Adjust CI to run a subset of validation if client.tsp is the only file changed

Need the ability to limit the Ci checks that are done when only the client.tsp is changed. This file does not affect the service at all and therefore shouldn't need all of the breaking change and api stewardship checks that happen when a service specification is changed.

Alternatively we could manage this file in the language independent azure-sdk repository, but that adds yet another repository involved when trying to generate our sdks from typespec.