ext authz: add dns san support for ext authz service (#7948)

Adds support for DNS SAN in ext authz peer validation

Risk Level: Low
Testing: Added
Docs Changes: Added
Release Notes: N/A

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

Mirrored from https://github.com/envoyproxy/envoy @ d4dc0a5b74acd42aafaaef8bb9c4ebed832674bd

envoy/service/auth/v2/attribute_context.proto

@@ -50,8 +50,8 @@ message AttributeContext {
     // The authenticated identity of this peer.
     // For example, the identity associated with the workload such as a service account.
     // If an X.509 certificate is used to assert the identity this field should be sourced from
-    // `Subject` or `Subject Alternative Names`. The primary identity should be the principal.
-    // The principal format is issuer specific.
+    // `URI Subject Alternative Names`, `DNS Subject Alternate Names` or `Subject` in that order.
+    // The primary identity should be the principal. The principal format is issuer specific.
     //
     // Example:
     // *    SPIFFE format is `spiffe://trust-domain/path`