[TypeSpec Authoring Tool] Customize client.tsp to mitigate SDK breaking change

Customizing client.tsp can:
- Preserve existing class, method, and property names even when the service API changes.
- Hide or suppress generated types or members that would introduce breaking changes.
- Replace generated types with alternate shapes that match your existing SDK.
- Control client naming, access modifiers, paging behavior, and more.

It is essential to ensure that the TypeSpec Authoring Tool:
- comprehends fundamental customization mechanisms,
- recommends appropriate customization approaches for various scenarios.

Possible Scenarios (to inspire requriements)
- "What did I just break?" Trigger updating client.tsp and running a validation
- "I want to add pagination to this list operation without breaking existing clients"
- "Review the specs from an SDK perspective"
- "Give me a client.tsp template that minimizes breaking changes over time."