Add AsyncAPI spec for Realtime API

[HavenDV]

Specification: https://www.asyncapi.com/en
Example: https://github.com/AssemblyAI/assemblyai-api-spec/blob/main/asyncapi.yml

[StephenHodgson]

added in #356

[StephenHodgson]

Duplicate of #341

[HavenDV]

Although schemas have been added to OpenAPI spec and that's great, I think it still makes sense because the OpenAPI spec doesn't describe Realtime interactions, while the AsyncAPI spec is aimed at this and fully describes each step, and client code can be generated and maintained automatically as complexity increases.
This is supported by some generators already (e.g. fern - https://buildwithfern.com/), I also plan to support it in my generator - https://github.com/tryAGI/AutoSDK
This may also be a motivator for other generators