fuzz test: Ensure content type is valid. (#21123)

Signed-off-by: Andre Vehreschild <vehre@x41-dsec.de>

Mirrored from https://github.com/envoyproxy/envoy @ 84b31a3a51b1deedaa6f363d7e3609c8c9f9a736

envoy/config/core/v3/substitution_format_string.proto

@@ -106,7 +106,8 @@ message SubstitutionFormatString {
   //
   //   content_type: "text/html; charset=UTF-8"
   //
-  string content_type = 4;
+  string content_type = 4
+      [(validate.rules).string = {well_known_regex: HTTP_HEADER_VALUE strict: false}];
 
   // Specifies a collection of Formatter plugins that can be called from the access log configuration.
   // See the formatters extensions documentation for details.