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/payload.c b/src/payload.c
index 3d176d6..af4ae1c 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -22,7 +22,7 @@
 #include <haproxy/arg.h>
 #include <proto/channel.h>
 #include <proto/connection.h>
-#include <proto/sample.h>
+#include <haproxy/sample.h>
 #include <proto/http_ana.h>