Eventstream client integration

[bretambrose]

A bunch of miscellaneous CRT updates to support eventstream and Greengrass RPC.

• Some events have been renamed
• Added support for LiftedPromise, a promise whose resolve and reject methods are available to external actors. Primary use is to remove internal awaits in the promise body that may never resolve (based on network events, for example) and instead let external events drive resolution/rejection
• Added support for CancelController, a simple type with a cancel method that can be listened to for promise rejection

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