MINOR: http: Implement "early-hint" http request rules.

This patch implements http_apply_early_hint_rule() function is responsible of
building HTTP 103 Early Hint responses each time a "early-hint" rule is matched.
diff --git a/include/common/http.h b/include/common/http.h
index f876891..3b1620d 100644
--- a/include/common/http.h
+++ b/include/common/http.h
@@ -124,6 +124,7 @@
 extern const uint8_t http_char_classes[256];
 
 const struct ist HTTP_100;
+const struct ist HTTP_103;
 extern const char *HTTP_301;
 extern const char *HTTP_302;
 extern const char *HTTP_303;