ext_proc: Add code to check mutation rules (#19449)

Signed-off-by: Gregory Brail <gregbrail@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ab859de2d47341326b802c90a8388821cf9ebbc9

envoy/config/common/mutation_rules/v3/mutation_rules.proto

@@ -32,6 +32,10 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // tested against the rules here. Disallowed header mutations will be
 // ignored unless *disallow_is_error* is set to true.
 //
+// Attempts to remove headers are further constrained -- regardless of the
+// settings, system-defined headers (that start with ":") and the "host"
+// header may never be removed.
+//
 // In addition, a counter will be incremented whenever a mutation is
 // rejected. In the ext_proc filter, that counter is named
 // "rejected_header_mutations".