use Hyper as a HTTP backend

[bagder]

Status: https://github.com/curl/curl/wiki/Hyper

Made a draft PR early to make sure this work doesn't break the "normal" builds. expect rebases!

• make configure find Hyper
• initial code that creates and issues a request with Hyper
• use the correct method, path and default headers
• docs explaining how to build with Hyper
• receive/store response correctly
• debug callback for received content
• pass received data and headers to correct callbacks
• no memory-leaks
• works with HTTPS (if done with HTTP/1)
• tell Hyper to not lowercase headers
• debug callback for sent headers
• supports custom request headers
• HTTP/2
• curl -I (HEAD) works over HTTP(S) and h1/h2
• over proxies
• referer (-e )
• redirects
• cookies
• test suite "Hyper compatible"
• Basic auth headers
• Basic proxy auth headers
• simple POSTs work
• handle POST/PUT
• multi-part formpost
• send custom request headers without values
• pass on the "status reason" from the HTTP/1 status line
• provide Hyper version in curl_version_info function
• handle CURL_HTTP_VERSION_1_0
• chunked-encoded h1 "upload"
• debug callback for sent content
• multiplexed HTTP/2
• h2 Upgrade:
• HTTP auth
• pausing transfers
• co-exist with a HTTP/3 build
• receiving HTTP/1 trailers
• sending HTTP/1 trailers
• accept-encoding
• transfer encoding

[bagder]

I've cleaned up the commits now in preparation for the first merge of this work. My plan with this merge is to provide a first version that users can check out and try (opt-in of course), but to not affect the default build at all.

I will then continue fixing the Hyper use further in follow-up PRs, hopefully much smaller and less disruptive.