Changes:

• Adds an API ListProjectsForOrganizationAndUser that an org member with read_org_members permission can use to list the projects another org member has access to.
• Adds an API ListUsergroupsForOrganizationAndUser that an org member with read_org_members permission can use to list the user groups another org member belongs to.
• Adds support for optionally filtering by role in the following APIs:
  • ListOrganizationMemberUsers
  • ListOrganizationMemberUsergroups
  • ListProjectMemberUsers
  • ListProjectMemberUsergroups
• Added an include_counts option to the ListOrganizationMemberUsers API. If set, the result will contain two additional fields projects_count and usergroups_count for each member.
• Added an include_counts option to the ListOrganizationMemberUsergroups API. If set, the result will contain a users_count field for each user group.

Contributes to #6578

Frontend integration notes:

• To show the number of projects and groups that org members belong to, pass include_counts=true when calling ListOrganizationMemberUsers.
• When you want to list the projects or usergroups a specific org member belongs to, use ListProjectsForOrganizationAndUser and ListUsergroupsForOrganizationAndUser. Make sure to call these lazily when the org admin drills-down for it (it's too heavy to load these eagerly for all members in the listing at once).

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!