http: add support for forwarding peer cert chains (#7026)

Description: This adds support for forwarding peer cert chains via the XFCC header.

Risk Level: Low
Testing: Updated existing tests
Docs Changes: Updated XFCC doc
Release Notes: Added a note about the feature
Fixes #6135
Also see #6230

Signed-off-by: Venil Noronha <veniln@vmware.com>

Mirrored from https://github.com/envoyproxy/envoy @ 15abaec72b0374e29490e8e074751e6246980e87

envoy/config/filter/network/http_connection_manager/v2/http_connection_manager.proto

@@ -317,6 +317,7 @@ message HttpConnectionManager {
   ForwardClientCertDetails forward_client_cert_details = 16
       [(validate.rules).enum.defined_only = true];
 
+  // [#comment:next free field: 7]
   message SetCurrentClientCertDetails {
     // Whether to forward the subject of the client cert. Defaults to false.
     google.protobuf.BoolValue subject = 1;
@@ -328,6 +329,12 @@ message HttpConnectionManager {
     // Defaults to false.
     bool cert = 3;
 
+    // Whether to forward the entire client cert chain (including the leaf cert) in URL encoded PEM
+    // format. This will appear in the XFCC header comma separated from other values with the value
+    // Chain="PEM".
+    // Defaults to false.
+    bool chain = 6;
+
     // Whether to forward the DNS type Subject Alternative Names of the client cert.
     // Defaults to false.
     bool dns = 4;