Implemented Model Favoriting from Issue Feature: Add models to a favorite list

[alewtschuk]

I've gone ahead and implemented model favoriting within the model selector to fix the issue mentioned below. Still needs testing and trial to see if the feature is fully implemented.

Assuming the pull request and implementation is fine to proceed with as Issue 985 was flagged by and discussed with @meowgorithm in the original issue. If not please let me know and I'll adjust course!

Fixes #985

• [x] I have read CONTRIBUTING.md.
• [x] I have created a discussion that was approved by a maintainer (for new features).

[charmcli]

All contributors have signed the CLA ✍️ ✅
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[alewtschuk]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.