Add Group Service Accounts API

What does this MR do and why?

Describe in detail what your merge request does and why.

Fixes #407775 (closed)

Add new API endpoint to create service accounts within a group.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a group and ensure your test user is an owner.
2. Create a Personal Access Token with api scope.
3. Test the API via a terminal: curl -XPOST "https://gdk.test:3443/api/v4/groups/<group_id>/service_accounts?access_token=your_test_token"
4. Observe the API returns a response like:

   {"id":57,"username":"service_account_group_33_6018816a18e515214e0c34c2b33523fc","name":"Service account user","state":"active","avatar_url":"...","web_url":"https://gdk.test:3443/service_account_group_33_6018816a18e515214e0c34c2b33523fc"

Optionally call the API again with a non-existent group ID (returns 404 not found), or with an access token for a user that's not an owner in the group (returns forbidden).

MR acceptance checklist

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