Microsoft.ClientModel.TestFramework improvements

[m-redding]

Phase 1:

• Add README and additional samples
  • WIP improvements to unbranded test framework #53483
• Bring improvements over to Microsoft.ClientModel.TestFramework #53190

Phase 2:

• Upgrade NUnit to 4.x #53413

Phase 3:

• Unify Microsoft.ClientModel.TestFramework and Azure.Core.TestFramework #53412
• Testing updates: integration tests via fake client, additional unit tests, tests for asset sync
• Add support for "Azure concepts" in some form - resource bootstrapping, service versions, ...

Remaining:

• Add non-windows implementation to ProcessTracker #52513
• Add LRO polling reduction for perf boosts on LROs - TBD on this, OpenAI isn't really using LROs anymore, and there isn't a confirmed story for this yet in generated code
• Fix websocket recording?