MEDIUM: h2: send connect protocol h2 settings

In order to announce support for the Extended CONNECT h2 method by
haproxy, always send the ENABLE_CONNECT_PROTOCOL h2 settings. This new
setting has been described in the rfc 8441.

After receiving ENABLE_CONNECT_PROTOCOL, the client is free to use the
Extended CONNECT h2 method. This can notably be useful for the support
of websocket handshake on http/2.

include/haproxy/h2.h

@@ -163,6 +163,7 @@ enum h2_err {
 #define H2_SETTINGS_INITIAL_WINDOW_SIZE     0x0004
 #define H2_SETTINGS_MAX_FRAME_SIZE          0x0005
 #define H2_SETTINGS_MAX_HEADER_LIST_SIZE    0x0006
+#define H2_SETTINGS_ENABLE_CONNECT_PROTOCOL 0x0008
 
 
 /* some protocol constants */

src/mux_h2.c

@@ -1593,6 +1593,10 @@ static int h2c_send_settings(struct h2c *h2c)
 		chunk_memcat(&buf, "\x00\x02\x00\x00\x00\x00", 6);
 	}
 
+	/* rfc 8441 #3 SETTINGS_ENABLE_CONNECT_PROTOCOL=1
+	 * sent automatically */
+	chunk_memcat(&buf, "\x00\x08\x00\x00\x00\x01", 6);
+
 	if (h2_settings_header_table_size != 4096) {
 		char str[6] = "\x00\x01"; /* header_table_size */
 
@@ -2197,6 +2201,10 @@ static int h2c_handle_settings(struct h2c *h2c)
 				h2c->streams_limit = arg;
 			}
 			break;
+		case H2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
+			/* nothing to do here as this settings is automatically
+			 * transmits to the client */
+			break;
 		}
 	}