[Experiment] Cadl http webpubsub

[annelo-msft]

In progress: working description of the WebPubSub API in Cadl.

The goal of creating this Cadl description is to determine whether we have parity with the existing DPG v1.0 generator, which generates WebPubSub from swagger. At this point, only a few operations have been described.

Code generated to .NET Template project from this Cadl description can be found in this PR: Azure/azure-sdk-for-net#29499

Outstanding items:

• Using @summary tag causes a schema_violation error in generated swagger -- need to use @doc tag for parameter descriptions
• Need a way to specify hub is a client parameter: Provide a way to indicate that a parameter is a "client parameter" microsoft/typespec#664 -- for now, we are working around this with the swagger transform used in the sdk/webpubsub project.
• Would be nice to use OkResponse instead of Response<200>: Add untemplated OkResponse to cadl.http microsoft/typespec#666
• Is there a way to share a route description across operations on the same route?
• How to specify @consumes per operation? : Allow adding @produces and @consumes at the operation level microsoft/typespec#665
• Is there a need to handle x-ms-error-response in the Cadl definition?
• We can't use model to group input parameters, because we cannot add swagger transforms to $ref parameters