RPC endpoint for associating a function call's return value with a message queue

[tpmccallum]

The RPC caller provides Azure credentials in the HTTP request.

This operation will redirect the function's return value to a message queue. We should support

• Azure Queue Storage
• Azure Events Hub
• AWS SNS
• AWS SQS
• Twilio

[tpmccallum]

Caller can now set a full callback object for the wasm executable. This callback object can contain all of the request information which will allow the function's return value to be redirected to the details provided in the callback.
https://github.com/second-state/wasm-joey/blob/master/src/server.js#L1779

Here is an example of how a caller would create a POST request (the callback object) for an AWS SQS request.
https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-making-api-requests.html#structure-post-request

If a caller would prefer not to set the callback permanently, but rather per HTTP request, they can add the AWS SQS request in the header of their call. Joey will detect this and execute the callback request using the function's return value as the body/data. This option may be preferred when sending authentication for the AWS SQS request.
https://github.com/second-state/wasm-joey/blob/master/src/server.js#L1457