Add user-actor feature flag for pipelines

What does this MR do?

Adds a :graphql_pipeline_details_users flag to gate access to the new graph by user in addition to by project. We plan to start by making the new graph available to a number of individual users before enabling it by project and then move to certain projects while keeping the users enrolled.

Therefore we need both flags enabled at once and two flags.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #276949 (closed), #299112 (closed)