engine: record the transport used to submit

[bassosimone]

This issue is about helping the backend to figure out which transport has been used to submit a measurement. I am against adding this information directly inside the measurement because it strikes me as a bad idea to mutate the measurement to add an extra field just before submitting (it screams data race and I don't like it). So, my inclination here would be to add a header to the request that says which is the transport we're currently using. Then, we could teach the API or monitoring to know and record which is the transport that has been used.

[hellais]

This should be done once we have #2844