MINOR: http_htx: Add functions to manipulate HTX messages in http_htx.c

This file will host all functions to manipulate HTTP messages using the HTX
representation. Functions in this file will be able to be called from anywhere
and are mainly related to the HTTP semantics.
diff --git a/include/proto/http_htx.h b/include/proto/http_htx.h
new file mode 100644
index 0000000..1ca7902
--- /dev/null
+++ b/include/proto/http_htx.h
@@ -0,0 +1,40 @@
+/*
+ * include/types/http_htx.h
+ * This file defines function prototypes for HTTP manipulation using the
+ * internal representation.
+ *
+ * Copyright (C) 2018 HAProxy Technologies, Christopher Faulet <cfaulet@haproxy.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation, version 2.1
+ * exclusively.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#ifndef _PROTO_HTTP_HTX_H
+#define _PROTO_HTTP_HTX_H
+
+#include <common/buf.h>
+
+#include <types/h1.h>
+#include <types/http_htx.h>
+
+union h1_sl http_find_stline(const struct htx *htx);
+int http_find_header(const struct htx *htx, const struct ist name, struct http_hdr_ctx *ctx, int full);
+int http_add_header(struct htx *htx, const struct ist n, const struct ist v);
+int http_replace_reqline(struct htx *htx, const union h1_sl sl);
+int http_replace_resline(struct htx *htx, const union h1_sl sl);
+int http_replace_header_value(struct htx *htx, struct http_hdr_ctx *ctx, const struct ist data);
+int http_replace_header(struct htx *htx, struct http_hdr_ctx *ctx, const struct ist name, const struct ist value);
+int http_remove_header(struct htx *htx, struct http_hdr_ctx *ctx);
+
+#endif /* _PROTO_HTTP_HTX_H */