Make tunneled HTTP/2 CONNECT bodies easier to use

[bneradt]

Masakazu added support to ATS for HTTP/2 CONNECT methods and attempted adding an autest via Proxy Verifier here:
apache/trafficserver#9616 (comment)

While doing so he ran into issues with Proxy Verifier incorrectly requiring certain pseudo header fields for CONNECT requests. That issue is recorded in #252.

While working on that test, though, Masakazu observed that crafting HTTP/1.1 requests via raw DATA or content nodes is painful. It would be nice if they could be crafted using the otherwise available HTTP/1.1 yaml specification nodes in Proxy Verifier as works naturally well for HTTP/1. See for example:

https://github.com/apache/trafficserver/blob/f25206a24c74caae11e3cc6aa650036489cf5df3/tests/gold_tests/connect/replays/connect.replay.yaml#L42-L64