OpenAPI emitter: allow operationId to be set on interfaces and namespaces to act as prefix specification

[artsiommiksiuk]

Clear and concise description of the problem

@OpenAPI.operationId("SomeOperations")
namespace Some {
   op foo(): void;
}

This would result in foo having operationId = "SomeOperations_foo"

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• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[markcowl]

@artsiommiksiuk Note that exposing setOperationId may allow you to use your own decorator for this in the interim