MINOR: h2: implement H2->HTX request header frame transcoding

Till now we could only produce an HTTP/1 request from a list of H2
request headers. Now the new function h2_make_htx_request() does the
same but using the HTX encoding instead, while respecting the H2
semantics. The code is not much different from the first version,
only the encoding differs.

For now it's not used.
diff --git a/include/common/h2.h b/include/common/h2.h
index d75a3f4..e916732 100644
--- a/include/common/h2.h
+++ b/include/common/h2.h
@@ -32,6 +32,7 @@
 #include <common/config.h>
 #include <common/http-hdr.h>
 #include <common/ist.h>
+#include <proto/htx.h>
 
 
 /* indexes of most important pseudo headers can be simplified to an almost
@@ -154,6 +155,7 @@
 /* various protocol processing functions */
 
 int h2_make_h1_request(struct http_hdr *list, char *out, int osize, unsigned int *msgf);
+int h2_make_htx_request(struct http_hdr *list, struct htx *htx, unsigned int *msgf);
 
 /*
  * Some helpful debugging functions.