Add support for x-ms-client-name extension

[GunboatDiplomat]

There is currently no way to control the casing of properties in the generated model. For example, when using a C# Dictionary with enum keys on the backend, I need the generated model to preserve the original enum string casing used during serialization.

[abhey8]

Hi — I’d like to work on this and submit a PR implementing x-ms-client-name support in the TypeScript generator.
Plan: add generator logic to prefer the x-ms-client-name vendor extension as the serialized JSON property name, add a unit test demonstrating the generated serializer/deserializer uses that name, and open a draft PR.

If maintainers prefer any particular behavior or style - e.g. whether to support an option to enable/disable it - please let me know; otherwise I’ll follow the existing generator patterns.