Envoy ASSERT() due to regex rewrite path string contains

It is observed that Envoy crash in ASSERT() due to regex rewrite path string contains invalid character '
'.

We should prohibit null characters , 
,
 in the regex rewrite substitution string. This is well guarded in most other cases like RouteAction:prefix_rewrite, but is missing in RouteAction:regex_rewrite:substitution.

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 23f3ee57b012e28aea0d6e7bdd6a991edd392741

envoy/type/matcher/v3/regex.proto

@@ -95,5 +95,6 @@ message RegexMatchAndSubstitute {
   // backslash followed by the capture group number to denote a numbered
   // capture group. E.g., ``\1`` refers to capture group 1, and ``\2`` refers
   // to capture group 2.
-  string substitution = 2;
+  string substitution = 2
+      [(validate.rules).string = {well_known_regex: HTTP_HEADER_VALUE strict: false}];
 }