Specify a “callback url” in the header

[second-state]

Specify a “callback url” in the header

[tpmccallum]

This will be useful when executing a single function as per this endpoint /api/run/:wasm_id/:function_name

[juntao]

We should also support callback_url_headers. It is a JSON array of http headers. That is needed because the callback could require header-based authentication. Example:

https://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/index.html#-Authentication

[juntao]

Per our discussion on slack, we should NOT add this header. Instead, Joey should support a callback JSON element in the function return value (only applies when the function returns a string).

If this callback element is present, Joey will remove it from the result, and then send the result to the callback as the http request body. The callback element structure is as follows.

'callback': {
  'url': 'http://next.com/next',
  'method': 'POST',
  'headers': [
    'name1': 'value1',
    'name2': 'value2'
  ]
}

[tpmccallum]

Callback is now implemented.
Documentation is located at the following URL
https://web.postman.co/collections/4967217-84b93d68-2073-4f51-bde3-68f19de57c62?version=latest&workspace=6683d8e5-f543-4751-8871-aeceebf90811#2b37d947-1b86-4d51-b836-40359e8b92a4