tls: add support for private keys with passphrase (#5175)

This adds support for password encrypted private keys. The password is
to be supplied as a regular data source in the TlsCertificate
configuration.

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

Mirrored from https://github.com/envoyproxy/envoy @ 94eb347914fc5812ee35c1c2a66c1784579bfb87

envoy/api/v2/auth/cert.proto

@@ -73,7 +73,8 @@ message TlsCertificate {
   // The TLS private key.
   core.DataSource private_key = 2;
 
-  // [#not-implemented-hide:]
+  // The password to decrypt the TLS private key. If this field is not set, it is assumed that the
+  // TLS private key is not password encrypted.
   core.DataSource password = 3;
 
   // [#not-implemented-hide:]