Improve ImmutableArrayBuilder<T> type in MVVM Toolkit generators

[Sergio0694]

Overview

This PR includes a couple tweaks to ImmutableArrayBuilder<T>:

• Drops the ObjectPool<T> use, and just uses ArrayPool<T>. This means we can now also benefit from the automatic trimming that ArrayPool<T> implements if there is high GC pressure. Also, we're no longer reinventing the wheel here.
• Make the type a ref struct, since we're never meant to pass this outside or box it.
• Add ref scope annotations where needed

PR Checklist

• [X] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [X] Based off latest main branch of toolkit
• [X] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [X] Tested code with current supported SDKs
• [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [X] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [X] Contains NO breaking changes
• [X] Every new API (including internal ones) has full XML docs
• [X] Code follows all style conventions

[Sergio0694]

Hey @Youssef1313, would you mind taking a quick look when you have a minute? Thank you! 🙂