Include measurements in HTTP headers

[ameba23]

Closes #15

This is a big change as we switch from a protocol-agnostic TCP proxy to only allowing HTTP.

This example was helpful in setting up HTTP proxying using hyper.

Still TODO:

• Error handling
• Actually include the headers (not just forward requests / responses)
• Use HTTP 2 (or make an issue and do it in a separate PR)
• Add explanation in README and example header content (JSON)

Connection re-use (to save attestation handshakes for multiple requests) will be handled in separate PR.