Decide on mutability pattern for providers

[JoshLove-msft]

Some properties can be updated via Update methods, whereas others have public setters. Prior to TypeSpec GA, we should decide on a pattern and implement it across the various provider types.

[JoshLove-msft]

Plan is to use setters. Anytime a customer invokes a setter is guaranteed to be after the protected Build method was called.

[JoshLove-msft]

Chatted with @tg-msft, and the recommendation is to instead use Update methods as this gives us room for future optimizations in how we handle updates. It also fits with the paradigm that ASTs are immutable. We may want to return new instances from the Update method as well.