Add `all_available` argument to Groups GQL query

What does this MR do and why?

Add support for all_available argument to the Groups GQL query

References

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• Closes #505045 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

1. Sign in to your local GitLab instance with a non-admin user that is a member of at least one public group.
2. Go to http://localhost:3000/-/graphql-explorer.
3. Run the following GQL Query

query {
  groups(allAvailable: <boolean>) {
    nodes {
      name
      fullPath
    }
  }
}

4. When true, the query should return all public groups. Otherwise, it should only return the groups where the user is a member of.
5. Special Case: When unauthenticated, the query should always return all public groups regardless of the argument value (always treated as true).