Check for a Content-Type header on non-GET federation outgoing requests

Non-GET requests should have a Content-Type header (with `application/json` as its value). Sytest should check whether this header is included in outgoing federation requests and is set to the right value.

Start by locating Sytest's checks for outgoing federation requests and the existing handling of non-GET requests. Add coverage for the required Content-Type header and its application/json value, then run the relevant federation test suite to confirm the check passes.

Written by the indexing model from the issue text.