refactor: Rename CourseEditLTIFieldsEnabledFlag to CourseAllowPIISharingInLTIFlag and use it for LTI1.3
This commit renames the CourseEditLTIFieldsEnabledFlag to CourseAllowPIISharingInLTIFlag since the aim is to expand its scope to all LTI-related PII sharing. It also removes the current LTI1.3 waffle flag for PII sharing.

This moves the CourseEditLTIFieldsEnabledFlag from edx-platform to this repo. This allows using the flag from both lms and studio.