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

lib/http_proxy.h

@@ -25,34 +25,25 @@
  ***************************************************************************/
 
 #include "curl_setup.h"
+
+#if !defined(CURL_DISABLE_PROXY) && !defined(CURL_DISABLE_HTTP)
+
 #include "urldata.h"
 
-#if !defined(CURL_DISABLE_PROXY)
-
-#if !defined(CURL_DISABLE_HTTP)
 /* Default proxy timeout in milliseconds */
 #define PROXY_TIMEOUT (3600*1000)
 
-CURLcode Curl_conn_http_proxy_add(struct Curl_easy *data,
-                                  struct connectdata *conn,
-                                  int sockindex);
+void Curl_cf_http_proxy_get_host(struct Curl_cfilter *cf,
+                                 struct Curl_easy *data,
+                                 const char **phost,
+                                 const char **pdisplay_host,
+                                 int *pport);
 
 CURLcode Curl_cf_http_proxy_insert_after(struct Curl_cfilter *cf_at,
                                          struct Curl_easy *data);
 
 extern struct Curl_cftype Curl_cft_http_proxy;
 
-#endif /* !CURL_DISABLE_HTTP */
-
-CURLcode Curl_conn_haproxy_add(struct Curl_easy *data,
-                               struct connectdata *conn,
-                               int sockindex);
-
-CURLcode Curl_cf_haproxy_insert_after(struct Curl_cfilter *cf_at,
-                                      struct Curl_easy *data);
-
-extern struct Curl_cftype Curl_cft_haproxy;
-
-#endif /* !CURL_DISABLE_PROXY */
+#endif /* !CURL_DISABLE_PROXY  && !CURL_DISABLE_HTTP */
 
 #endif /* HEADER_CURL_HTTP_PROXY_H */