xds: Add Http11ProxyUpstreamTransport to MessagePrinter

[shivaspeaks]

Printer falls back to outputting the @error indicating that the type is not recognized. So because we are unpacking Http11ProxyUpstreamTransport from any, we need to add it to the registry.
Here is the example cds resource:

 "transportSocket": {
      "name": "envoy.transport_sockets.http_11_proxy",
      "typedConfig": {
        "@error": "envoy.extensions.transport_sockets.http_11_proxy.v3.Http11ProxyUpstreamTransport is not recognized; see @value for raw binary message data",
        "@type": "type.googleapis.com/envoy.extensions.transport_sockets.http_11_proxy.v3.Http11ProxyUpstreamTransport",
        "@value": ""
      }
    },

cc: @sergiitk

[sergiitk]

I also had .add(RawBuffer.getDescriptor()) in https://github.com/grpc/grpc-java/pull/12624/changes#diff-2131410ad09ff7bd1ee5c5071f816bc1a2eb3e66acc78659749a9bd00dfe94b1 - is this not needed?