REORG: include: move sample.h to haproxy/sample{,-t}.h

This one is particularly tricky to move because everyone uses it
and it depends on a lot of other types. For example it cannot include
arg-t.h and must absolutely only rely on forward declarations to avoid
dependency loops between vars -> sample_data -> arg. In order to address
this one, it would be nice to split the sample_data part out of sample.h.
diff --git a/src/server.c b/src/server.c
index 468814c..ae4f044 100644
--- a/src/server.c
+++ b/src/server.c
@@ -22,6 +22,7 @@
 #include <haproxy/dns.h>
 #include <haproxy/errors.h>
 #include <haproxy/namespace.h>
+#include <haproxy/sample.h>
 #include <haproxy/time.h>
 
 #include <types/applet.h>
@@ -37,7 +38,6 @@
 #include <haproxy/port_range.h>
 #include <haproxy/protocol.h>
 #include <proto/queue.h>
-#include <proto/sample.h>
 #include <proto/server.h>
 #include <proto/stream.h>
 #include <proto/stream_interface.h>