Add API to activate memberships

What does this MR do and why?

This API will be used for the Usage Quota page to toggle the membership state on free groups. It changes the membership states for all related memberships of a user to the given group to active. It also makes sure that free namespaces can't activate members when they are at limit when the feature flag is enabled.

Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/352638

The opposite API endpoint (to de-activated) is implemented at: !85685 (merged)

Screenshots or screen recordings

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How to set up and validate locally

1. Get an access token http://localhost:3000/-/profile/personal_access_tokens that allows modification via API
2. Start the gdk with

   export GITLAB_SIMULATE_SAAS=1
   gdk start

Test activation

1. Find a group and user that you want to test it on - make sure that the Group has a free or default plan (required for testing the limit)

   g = Group.first # or any other Group ID you want to test it on
   puts g.id # get the group ID for the API call
   puts g.members.last.user.id # get a user ID for the API call

2. Deactivate the user's membership

   g.members.last.wait
   g.members.last.awaiting? # => should output `true`

3. Execute the curl request for a user within the group

    curl --request PUT --header "PRIVATE-TOKEN: TOKEN" "http://localhost:3000/api/v4/groups/GROUP_ID/billable_members/USER_ID/activate"

4. Check the user's membership status now

   g.members.last.awaiting? # => should be `false`
   g.members.last.active? # => should be `true`

Testing limit

1. Now let's deactivate the user again and enable free_user_cap for the Group:

   g.members.last.wait
   ::Feature.enable(:free_user_cap, g)

2. Add members to the group so it's at least 5 members: http://localhost:3000/groups/GROUP_NAME/-/group_members
3. Try activating our user again:

    curl --request PUT --header "PRIVATE-TOKEN: TOKEN" "http://localhost:3000/api/v4/groups/GROUP_ID/billable_members/USER_ID/activate"

4. You should see the error Bad request - You reached the user limit

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.