Proposal: Support for callback_url

[susheel]

We would like to implement a mechanism to allow Task submitters to specify a callback_url that will be used by TES to POST any change in status to the callback_url.

Requirements:

• Task submitter must be able to specify a single callback_url as part of the TesRequest
• TES will POST the full TesResponse to the callback_url on change in status using any (optional) auth headers

Related Issue: WES#133

[uniqueg]

One thing to think about is whether the task submitter/client should be required/encouraged to send a secret alongside the callback_url that could then be sent back to the client together with the TesResponse. This might help prevent third parties from messing with task states.

[uniqueg]

PR available: #176