Add ability to send custom identifiers with web socket messages

[designatednerd]

Feature request

The current state of our RequestBodyCreator's interaction with our web socket library's OperationMessage does not allow the user to set an ID on the operation message itself. This limits what can be done with each message since some servers rely on the ID of the operation message being something specific, rather than the current setup which uses the number in the sequence of messages as the ID.

We should add the ability to have the id on OperationMessage have a specified value.

Motivation

This will facilitate supporting servers which require a custom identifier to be passed along with the subscriptions, specifically AWS AppSync subscriptions.

See more extensive discussion here

Outstanding Questions

• Right now the ID being sent is number in the sequence of messages. It's not 100% clear what's depending on this at the moment, but I suspect there may be some stuff around retries and/or cancellation that might be dealing with this. We will need to identify exactly what's going on here before making this customizable.

[cmcnally-beachbody]

@designatednerd Any plans to to make this update anytime soon?