Migrate Key Vault client generation to TypeSpec

[chlowell]

This issue tracks migrating Key Vault client generation from OpenAPI to TypeSpec:

• [ ] administration
• [ ] certificates
• [ ] keys
• [ ] secrets

This is the high-level process for each library:

• add language-specific configuration to the library's tspconfig.yaml in the azure-rest-api-specs repo. See these PRs:
  • https://github.com/Azure/azure-rest-api-specs/pull/30239
  • https://github.com/Azure/azure-rest-api-specs/pull/28708
  • https://github.com/Azure/azure-rest-api-specs/pull/29248
  • https://github.com/Azure/azure-rest-api-specs/pull/29249
• check tsp-client setup for SDK repo (docs)
• add tsp-location.yaml for the library (docs, example)
• generate code with tsp-client
• add customization to client.tsp (example)