Setup helper methods to pass data to Vue app

What does this MR do?

Related to #281809 (closed)

This MR sets up the helper methods and serializers/entities needed to pass the project members data from the backend to the frontend. This is needed to complete &4901 (closed). These helper methods will be used by the frontend in a subsequent MR. A similar change was done for the group members in &4233 (closed) (app/helpers/groups/group_members_helper.rb)

Summary of changes

1. Move properties shared between group_group_link and project_group_link to a group_link entity
2. Change group_group_link_entity.rb to extend group_link_entity.rb
3. Create a project_group_link_entity.rb file that extends group_link_entity.rb
4. Create project_members_list_data_attributes and project_group_links_list_data_attributes helpers that will be used to pass data to the Vue frontend.

Screenshots (strongly suggested)

No visual changes. For context, this is what the frontend will look like:

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, backstage change
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team