Create a hasEtag in an operation exposed by TCGC

[lmazuel]

Feels like right now, we require emitters to manually sniff header names to decided if an operation required AzureCore MatchCondition. That's how I read that Python code for instance:

https://github.com/microsoft/typespec/blob/eaa71125e74cb867ba4fa69eb28f4e091e8a33d5/packages/http-client-python/generator/pygen/preprocess/__init__.py#L299-L330

I believe it would be beneficial, if we were to raise up that computation to TCGC operation type, to simplify emitter work to implement (in particular here C++ emitter).

Thoughts?