[tcgc] add overrideClientMethod decorator

[iscai-msft]

Clear and concise description of the problem

We want the ability in client.tsp to override a generated client method. When we switch to ... always meaning spread, we will also need the ability to generate options bags for languages that prefer options bags. Adding a decorator like overrideClientMethod will allow authors to clearly state their preferred signature for specific language(s). This will also allow patched customizations without having to craft an entire client to modify one operation

// main.tsp
namespace MyService;

model Params = {
  foo: string;
  bar: string;
}

op myOperation(...Params): void;

// client.tsp
namespace MyCustomizations;

op myOperationCustomization(@bodyRoot params: Params): void;

@@overrideClientMethod(MyService.myOperation, MyCustomizations.myOperationCustomization);

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[weidongxu-microsoft]

link a design discussion for options bag #831
currently the mapping of model properties could be a challenge.

[weidongxu-microsoft]

A few questions:

1. Do we discard the serviceOperation design? It is kind of opposite to this design @serviceOperation(serviceOp) clientOp where here @overrideClientMethod(clientOp) serviceOp

2. Should we recommend dev to use ref to property (Timothee kind of recommend this)? E.g.

model Params = {
  foo: MyService.myOperationModel.foo;
  bar: MyService.myOperationModel.bar;
}

[iscai-msft]

1. We pretty much do. We make sure that the two operations result in the same exact service operation, then just use the overridden client method.
2. Yeah that's a good idea, this way we more clearly map things from the service to the client