Provide design for simple Cadl API description

[annelo-msft]

Provide design for #2331

CRUD ops on simple model using Cadl.Http description.

1. Provide Cadl file describing:

• Read (GET)
  • https://github.com/Azure/autorest.testserver/blob/cadl/cadl/http/op-crud/op-crud.cadl#L24
• Create (PUT)
• Update (PATCH)
• Delete (DELETE)

2. Provide expected C# output from DPG generator:

• Read (GET)
  • Async public method
  • Sync public method
  • Request creation method
• Create (PUT)
• Update (PATCH)
• Delete (DELETE)

Update

Working on describing WebPubSub in this PR: Azure/autorest.testserver#392

[annelo-msft]

Closing in favor of #2395