admin: add cert expiry details to certs admin end point (#4804)

Adds certificate expiry details to certs admin end point

Risk Level: Low
Testing: Automated tests

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

Mirrored from https://github.com/envoyproxy/envoy @ f3033ce27689e55052909598140a561172c490ba

envoy/admin/v2alpha/certs.proto

@@ -2,6 +2,8 @@ syntax = "proto3";
 
 package envoy.admin.v2alpha;
 
+import "google/protobuf/timestamp.proto";
+
 // [#protodoc-title: Certificates]
 
 // Proto representation of certificate details. Admin endpoint uses this wrapper for `/certs` to
@@ -33,6 +35,12 @@ message CertificateDetails {
 
   // Minimum of days until expiration of certificate and it's chain.
   uint64 days_until_expiration = 4;
+
+  // Indicates the time from which the certificate is valid.
+  google.protobuf.Timestamp valid_from = 5;
+
+  // Indicates the time at which the certificate expires.
+  google.protobuf.Timestamp expiration_time = 6;
 }
 
 message SubjectAlternateName {