proxy: http2 proxy tunnel implementation

- currently only on debug build and when env variable
CURL_PROXY_TUNNEL_H2 is present.
- will ALPN negotiate with the proxy server and swit

proxy: http2 proxy tunnel implementation

- currently only on debug build and when env variable
CURL_PROXY_TUNNEL_H2 is present.
- will ALPN negotiate with the proxy server and switch
tunnel filter based on the protocol negotiated.
- http/1.1 tunnel code moved into cf-h1-proxy.[ch]
- http/2 tunnel code implemented in cf-h2-proxy.[ch]
- tunnel start and ALPN set remains in http_proxy.c
- moving all haproxy related code into cf-haproxy.[ch]

VTLS changes
- SSL filters rely solely on the "alpn" specification they
are created with and no longer check conn->bits.tls_enable_alpn.
- checks on which ALPN specification to use (or none at all) are
done in vtls.c when creating the filter.

Testing
- added a nghttpx forward proxy to the pytest setup that
speaks HTTP/2 and forwards all requests to the Apache httpd
forward proxy server.
- extending test coverage in test_10 cases
- adding proxy tests for direct/tunnel h1/h2 use of basic auth.
- adding test for http/1.1 and h2 proxy tunneling to pytest

Closes #10780

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