Bug 1944522 - Add Content-Type header option in Harbormaster HTTP Build step

[La0]

Bug 1944522

This PR adds an option to the Harbormaster HTTP Build Step to send an extra Content-Type header.

The current behaviour is preserved by default: no Content-Type header is sent.

I was able to test these changes locally, querying my own machine through a public IP from the docker container.
Using a debug server, I logged the headers sent by Phabricator when triggering manually a build plan:

Accept: */*
Content-Type: application/json;test-bastien
Accept-Encoding: gzip
Content-Length: 0

[La0]

I rebased on top of #47 to get a CircleCI run