Improve #2434 (Add HTTP Headers to Webhook Agent)

[knu]

This PR is based on and adds some improvements to #2434. Here's a list of what I've done:

• Use the real header names instead of those in the HTTP_FOO_BAR format
• Extract a logic for including a header hash in event payloads from PostAgent, with the event_headers_style option
• Rename headers to event_headers (and accept an array of strings also)
• Rename header_key to event_headers_key
• Introduce EventHeadersConcern which implements all above to be shared among PostAgent and WebhookAgent
  • I'm also working on making ImapAgent use it to add email headers to its event payloads.
• Squash the commits into one removing file mode changes and whitespace diffs

/cc @Fishwaldo

[Fishwaldo]

looks good to me

[dsander]

Look good to me 👍

[knu]

Thanks!