Forward proxy-protocol correctly to tls backends

[jrobsonchase]

Currently, we ignore proxy protocol settings in our tunnels. This means that,
for tls and https connections that are terminated at the ngrok edge, the
incoming connections look like ProxyProtoStream(PlainTextStream). When we
then terminate tls to the backend/upstream/whatever you want to call it, the
final stream looks like TlsStream(ProxyProtoStream(PlainTextStream)), where
it's expected to look like ProxyProtoStream(TlsStream(PlainTextStream)),
since proxy-protocol is a lower layer than TLS.

To that end, this change plumbs the proxy-proto setting through to the endpoint
connection object, reads the header out of the stream, and then makes sure to
write that to the upstream first before terminating TLS.

[jrobsonchase]

Current dependencies on/for this PR:

• main
  • PR Forward proxy-protocol correctly to tls backends #112 👈

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[bobzilladev]

lgtm. double-checked proxy_protocol is advancing the buffer during parse.

[jrobsonchase]

Mmm, I'm using the parse error wrong. Gonna try to isolate the header reading logic to better test it.

[bobzilladev]

Think I've wrapped my head around what proxy_proto is up to now. Left some comments, but nothing that's a hard blocker.