MINOR: conn-stream: Add header file with util functions related to conn-streams

cs_utils.h header file will contain all util functions related to the
conn_streams. For now, few functions were added, all are equivalent to SI
functions. Idea is to progressively replace SI functions by CS ones.
diff --git a/include/haproxy/cs_utils.h b/include/haproxy/cs_utils.h
new file mode 100644
index 0000000..58334b8
--- /dev/null
+++ b/include/haproxy/cs_utils.h
@@ -0,0 +1,76 @@
+/*
+ * include/haproxy/cs_utils.h
+ * This file contains conn-stream util functions prototypes
+ *
+ * Copyright 2022 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 _HAPROXY_CS_UTILS_H
+#define _HAPROXY_CS_UTILS_H
+
+#include <haproxy/api.h>
+#include <haproxy/buf-t.h>
+#include <haproxy/channel-t.h>
+#include <haproxy/stream-t.h>
+#include <haproxy/task-t.h>
+#include <haproxy/conn_stream.h>
+
+/* returns the channel which receives data from this conn-stream (input channel) */
+static inline struct channel *cs_ic(struct conn_stream *cs)
+{
+	struct stream *strm = __cs_strm(cs);
+
+	return ((cs->flags & CS_FL_ISBACK) ? &(strm->res) : &(strm->req));
+}
+
+/* returns the channel which feeds data to this conn-stream (output channel) */
+static inline struct channel *cs_oc(struct conn_stream *cs)
+{
+	struct stream *strm = __cs_strm(cs);
+
+	return ((cs->flags & CS_FL_ISBACK) ? &(strm->req) : &(strm->res));
+}
+
+/* returns the buffer which receives data from this conn-stream (input channel's buffer) */
+static inline struct buffer *cs_ib(struct conn_stream *cs)
+{
+	return &cs_ic(cs)->buf;
+}
+
+/* returns the buffer which feeds data to this conn-stream (output channel's buffer) */
+static inline struct buffer *cs_ob(struct conn_stream *cs)
+{
+	return &cs_oc(cs)->buf;
+}
+/* returns the stream's task associated to this conn-stream */
+static inline struct task *cs_strm_task(struct conn_stream *cs)
+{
+	struct stream *strm = __cs_strm(cs);
+
+	return strm->task;
+}
+
+/* returns the conn-stream on the other side. Used during forwarding. */
+static inline struct conn_stream *cs_opposite(struct conn_stream *cs)
+{
+	struct stream *strm = __cs_strm(cs);
+
+	return ((cs->flags & CS_FL_ISBACK) ? strm->csf : strm->csb);
+}
+
+
+#endif /* _HAPROXY_CS_UTILS_H */