feat(151, oas): add an openapi extension for x-aep-long-running-operation

[toumorokoshi]

Clients will have to do significant inference around things like response types without additional documentation the client.

This change adds the minimally required information, but is designed to allow additional properties (perhaps things like timeouts) in the schema.

[toumorokoshi]

It looks like the protobuf world already has a pattern here, including the "response_type" extension:

rpc WriteBook(WriteBookRequest) returns (aep.api.Operation) {
    option (google.api.http) = {
      post: "/v1/{parent=publishers/*}/books:write"
      body: "*"
    };
    option (aep.api.operation_info) = {
      response_type: "WriteBookResponse"
      metadata_type: "WriteBookMetadata"
    };
  }

I think we're going to need to reconcile this.

At minimum, the values contained in both annotations must be the same so we can map them 1-1.

Additionally, it would be great if the names themselves could match. however, that seems difficult to do if we are going to be idiomatic, as the concept of a "type" in proto (really the message) would be called a "schema" in the json-schema / openapi world.

[toumorokoshi]

closing in favor of #27, which works off of a fork.