Align REQUIRED declaration with reality (#2946)

The `REQUIRED` proto label currently does nothing. As a result, some of
them are misaligned with reality -- fields marked REQUIRED are actually
not required, and often even have valid behaviors and are used in the
real world.

Unfortunately, many of these are fields that *should* be required but
are not. To maintain backwards compatibility, these cannot be made
required at this point, sadly.

In a future PR, I intend to make `REQUIRED` *actually make it required*,
so this PR is a pre-requisite to doing that.

Author: howardjohn

networking/v1alpha3/destination_rule.pb.go

@@ -1075,7 +1075,7 @@ message ClientTLSSettings {
 
   // Indicates whether connections to this port should be secured
   // using TLS. The value of this field determines how TLS is enforced.
-  TLSmode mode = 1 [(google.api.field_behavior) = REQUIRED];
+  TLSmode mode = 1;
 
   // REQUIRED if mode is `MUTUAL`. The path to the file holding the
   // client-side TLS certificate to use.