feat: add course flag to send external_user_id as user_id in LTI 1.1 XBlock launches

This commit introduces a new CourseWaffleFlag lti_consumer.enable_external_user_id_1p1_launches. When this flag is enabled for a course, LTI 1.1 XBlock launches in that course will send the user's external_user_id as the user_id attribute of the launch. external_user_id is the user's external user ID as defined, created, and stored by the external_user_ids Djangoapp in the edx-platform. When this waffle is not enabled for a course - the default case - LTI 1.1 XBlock launches in that course will continue to send the user's anonymous_user_id as the user_id attribute of the launch, as before.

This provides an opt-in opportunity for courses to send a consistent, static, and opaque user identifier in an LTI 1.1 XBlock launch. This may be necessary for integration with LTI tools that require such an identifier.

Please be aware that toggling this flag in a running course carries the risk of breaking the LTI integrations in the course. This flag should also only be enabled for new courses in which no LTI attempts have been made.