docs: New ADR to decide the future of the CourseEditLTIFieldsEnabledFlag

This ADR proposes that we use the existing CourseEditLTIFieldsEnabledFlag
for LTI Course tabs in addition LTI XBlocks, and move it to this repo as
intended via EDUCATOR-121.

docs/decisions/0005-lti-pii-sharing-flag.rst

+Unified Flag for Enabling Sharing of PII in LTI
+-----------------------------------------------
+
+Status
+======
+
+Proposal
+
+Context
+=======
+
+LTI integrations can be enhanced by sharing some level of personal information
+about users, such as their username and email. Sharing such information allows
+for a smoother registration flow among other benefits.
+
+The `LTI XBlock has options
+<https://github.com/edx/xblock-lti-consumer/blob/edec2a68282a2a1fc2b2036e1989d60688fa6b19/lti_consumer/lti_xblock.py#L471-L487>`_
+called ``ask_to_send_username`` and ``ask_to_send_email`` that enable sending
+such PII to the external tool. However, `by default these options are not
+visible
+<https://github.com/edx/xblock-lti-consumer/blob/edec2a68282a2a1fc2b2036e1989d60688fa6b19/lti_consumer/lti_xblock.py#L588-L599>`_
+to course authors, and there is no way to send PII to LTI tools. In order to
+even enable these options to appear in the LTI XBlock configuration, you need
+to first set a `configuration flag
+<https://github.com/edx/edx-platform/blob/e19ba34f5a564285b3a20a7298c20ca640ca5aa0/cms/djangoapps/xblock_config/models.py#L38-L86>`_
+via Django admin in studio.
+
+This configuration flag unlocks the editing of the above fields after which
+those fields can be set to allow sharing of PII. The configuration flag works
+on a course-wide level, while the flags above work on a per-LTI-XBlock-basis.
+
+With the recent addition of LTI Course tabs, there is need for a similar
+mechanism for enabling sending of PII. What needs to be determined is whether
+we should use the same flag for the purpose, or create a new flag for this use
+with LTI tabs.
+
+The arguments for creating a new flag are:
+
+1. The scope of the current flag doesn't match the new use case.
+2. Currently the flag is implemented in studio, while the flag now needs to be
+   used in LMS and MFEs.
+3. Keeping flags separate allows controlling this flag separately for course
+   tabs vs XBlocks.
+
+The arguments for using the same flag are:
+
+1. As part of `EDUCATOR-121 <https://openedx.atlassian.net/browse/EDUCATOR-121>`_
+   this config flag is already supposed to be moved to the LTI XBlock, which
+   would make it usable from LMS and Studio.
+2. The name and scope can both be changed while retaining the data.
+3. It's confusing to have two flags with very similar purposes.
+4. There is already a double layer of flags here. Enabling this flag will enable
+   configuring other option which in turn enable sending PII data. Adding
+   another separate flag doesn't help the situaiton. Even if the flag is only
+   needed for either tabs or XBlocks, there is another step to enabling sharing
+   PII which can simply be skipped if undesired.
+
+Decision
+========
+
+Given the above context it makes sense to move the existing flag to the LTI
+XBlock while making sure it uses the same data table. This move means that the
+flag will now be accessible via LMS and Studio, which is what is needed.
+
+The LTI course tabs code will be modified to check this value before allowing
+the sending of PII even if the tab's configuration allows sending PII.
+
+Currently, if this flag is enabled for a course, and the `ask_to_send_*` options
+are enabled for an XBlock, disabling this flag will not stop the XBlock from
+sending PII. This flag will *only* disable editing the `ask_to_send_*` options.
+This might not be desirable behavior. So we can also modify the scope of this
+flag to be the final arbiter of whether PII sharing is allowed or not instead
+of just controlling if those fields are editable.
+
+Additionally, any PII sharing via LTI1.3 can also be folded under the same flag.
+
+
+Consequences
+============
+
+If this proposal is implemented, there will be one consistent flag
+(``CourseEditLTIFieldsEnabledFlag``) that can be set for a course to
+enable/disable sharing of PII via LTI. This flag will apply for all LTI
+XBlocks, and for all LTI course tabs (currently only Discussions LTI tab).
+
+Additionally, if ``CourseEditLTIFieldsEnabledFlag`` is enabled for a course and
+subsequently disabled, then existing sharing of PII via LTI will stop across
+all LTI versions used by any course tab or LTI XBlock.
+
+A quick breakdown of how different options will work together once this
+proposal is implemented:
+
+- **CourseEditLTIFieldsEnabledFlag**: When this flag is disabled for a course
+  (the default), course authors will not see any options to send PII and no PII
+  will be sent by any LTI tool embedded in the course using course tabs or the
+  official LTI XBlock. If this flag was once enabled, and PII sharing options
+  were also enabled, they will be **overridden** by this flag.
+
+- **ask_to_send_username** and **ask_to_send_email**: If the above flag is
+  enabled the course author will see a UI with these options in the LTI
+  Course block (and course LTI tab configuration once that UI for that is
+  available). These optons can only be set if the above flag is enabled,
+  otherwise API operations to enable them should fail and their value will be
+  overridden to disabled.