This diff changes the data format to prefer "udp" to "quic" everywhere we were previously using "quic".
Previously, the code inconsistently used "quic" for operations where we knew we were using "quic" and "udp" otherwise (e.g., for generic operations like ReadFrom).
While it would be more correct to say that a specific HTTP request used "quic" rather than "udp", using "udp" consistently allows one to see how distinct events such as ReadFrom and an handshake all refer to the same address, port, and protocol triple. Therefore, this change makes it easier to programmatically unpack a single measurement and create endpoint stats.
Before implementing this change, I discussed the problem with @hellais who mentioned that ooni/data is not currently using the "quic" string anywhere. I know that ooni/pipeline also doesn't rely on this string. The only users of this feature have been research-oriented experiments such as urlgetter, for which such a change would actually be acceptable.
See https://github.com/ooni/probe/issues/2238 and https://github.com/ooni/spec/pull/262.
In a pure step-by-step model, we don't need to trace HTTP round trips like we did before. We _may_ want in the future to also have some form of HTTP tracing (see https://github.com/ooni/probe-cli/pull/868 for a prototype) but doing that is currently not in scope for moving forward the step-by-step design. For this reason, I only added a public convenience function for formatting an OONI spec compatible request. I also added new fields, which should be documented inside the ooni/spec repository (see https://github.com/ooni/probe/issues/2238).
Required by https://github.com/ooni/probe/issues/2237
