[.NET Analyzer] Scope AZC0020 to allow use in generated Patch Models

Per comment here: https://github.com/Azure/azure-sdk-for-net/pull/38159#discussion_r1294907876

We may need to wait until the JSON Merge Patch model pattern is finalized to understand how to detect the cases to enable without allowing others through.

Start with the .NET Analyzer rule AZC0020 and review the linked pull-request discussion. First understand how the JSON Merge Patch model pattern is finalized and how generated Patch Models can be identified. Done means AZC0020 permits the intended generated Patch Models without allowing other cases through.

Written by the indexing model from the issue text.