enhancement(http-server): add response_body_key for setting http response body

[frankh]

trafficstars

backwards compatible change to add a response_body_key option for the http-server source.

If specified, returns the json encoding of the given key in the event as the response body

fixes #21013

[bits-bot]

All committers have signed the CLA.

[frankh]

@jszwedko any interest in this PR?

we want this to be able to return a uuid to the client to be able to track an event, feels like a reasonably common use-case