tcp_proxy: support command operators in tunneling_config.hostname (#21067)

Signed-off-by: Jacek Ewertowski jacek.ewertowski1@gmail.com

Commit Message: tcp_proxy: support command operators in tunneling_config.hostname
Additional Description: This change enables dynamically setting tunneling_config.hostname with command operators.
This pull request is an alternative for auto_sni.
This change allows to configure TCP proxy as follows:
```
tunneling_config:
  hostname: %REQUESTED_SERVER_NAME%:443
```
Risk Level: Low
Testing: added unit tests
Docs Changes: done
Release Notes: done
Platform Specific Features: none
Fixes #19612
Fixes #21804

Mirrored from https://github.com/envoyproxy/envoy @ 764a2e9fbb06e2f27fd6775fdc0ed78313b94157

envoy/extensions/filters/network/tcp_proxy/v3/tcp_proxy.proto

@@ -69,6 +69,22 @@ message TcpProxy {
         "envoy.config.filter.network.tcp_proxy.v2.TcpProxy.TunnelingConfig";
 
     // The hostname to send in the synthesized CONNECT headers to the upstream proxy.
+    // This field evaluates command operators if set, otherwise returns hostname as is.
+    //
+    // Example: dynamically set hostname using downstream SNI
+    //
+    // .. code-block:: yaml
+    //
+    //    tunneling_config:
+    //      hostname: "%REQUESTED_SERVER_NAME%:443"
+    //
+    // Example: dynamically set hostname using dynamic metadata
+    //
+    // .. code-block: yaml
+    //
+    //    tunneling_config:
+    //      hostname: "%DYNAMIC_METADATA(tunnel:address)%"
+    //
     string hostname = 1 [(validate.rules).string = {min_len: 1}];
 
     // Use POST method instead of CONNECT method to tunnel the TCP stream.