REORG: mux-quic: export HTTP related function in a dedicated file

Extract function dealing with HTX outside of MUX QUIC. For the moment,
only rcv_buf stream operation is concerned.

The main objective is to be able to support both TCP and HTTP proxy mode
with a common base and add specialized modules on top of it.

This should be backported up to 2.6.
diff --git a/Makefile b/Makefile
index 08d0804..12a3c14 100644
--- a/Makefile
+++ b/Makefile
@@ -646,7 +646,7 @@
                 src/cbuf.o src/qpack-dec.o src/qpack-tbl.o src/h3.o src/qpack-enc.o \
                 src/hq_interop.o src/cfgparse-quic.o src/quic_loss.o \
                 src/quic_tp.o src/quic_stream.o src/quic_stats.o src/h3_stats.o \
-                src/quic_cc_cubic.o src/qmux_trace.o
+                src/quic_cc_cubic.o src/qmux_trace.o src/qmux_http.o
 endif
 
 ifneq ($(USE_LUA),)