Feature: Allow AsyncApi attribute on Interfaces

[ConstantinosSocratous]

Description

We are trying to auto generate an AsyncApi document spec from a strongly typed signalR server. (see https://github.com/essencebit/SignalRSwaggerGen/wiki)
That is, all the exposed methods will exists only in an interface instead of a class.

Adding the ability to add the AsyncApi attribute on an interface class with the Channel and Publish/Subscribe Operation attributes on the interface methods would be very beneficial.

Thanks