Add subgroup autocomplete for Group-level Protected Environments

What does this MR do and why?

This MR is a vertical slice for Group-level UI for Protected Environment settings feature. It adds the following changes:

• Add auto-complete endpoint for fetching subgroups of a given group.
• Renaming InvitedGroupSerializer to GroupSerializer for the consistency.

Screenshots or screen recordings

You can see the walk-through video for the full details, or the PoC MR about the prospect code changes to complete the feature.

How to set up and validate locally

You can see the walk-through video for the full details, or the PoC MR about the prospect code changes to complete the feature.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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