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

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/connect.c b/lib/connect.c
index 2dd4e66f2..2d940972e 100644
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -59,6 +59,7 @@
 #include "strerror.h"
 #include "cfilters.h"
 #include "connect.h"
+#include "cf-haproxy.h"
 #include "cf-https-connect.h"
 #include "cf-socket.h"
 #include "select.h"