thrift_proxy: introduce header transport (#4082)

Implements the Thrift "header" transport which primarily provides
key/value pairs that may contain arbitrary message metadata.

*Risk Level*: low
*Testing*: added unit/integration tests
*Docs Changes*: n/a
*Release Notes*: n/a

Signed-off-by: Stephan Zuercher <stephan@turbinelabs.io>

Mirrored from https://github.com/envoyproxy/envoy @ 01f403ec4ccef1ba47efbac6741106602ff78ca2

envoy/config/filter/network/thrift_proxy/v2alpha1/thrift_proxy.proto

@@ -23,6 +23,9 @@ message ThriftProxy {
 
     // The Thrift proxy will assume the client is using the Thrift unframed transport.
     UNFRAMED = 2;
+
+    // The Thrift proxy will assume the client is using the Thrift header transport.
+    HEADER = 3;
   }
 
   // Supplies the type of transport that the Thrift proxy should use. Defaults to `AUTO_TRANSPORT`.