TrpRoutingRequest: add and test routing requests

Initial commit introducing TrpRoutingRequest, TrpRoutingRequestBuilder.
Also provides a sets of automated tests

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Differential Revision: https://phabricator.apertis.org/D4629