thrift: implement Twitter protocol variant (#4363)

Implements the Twitter variant of the Thrift binary protocol,
as implemented by the finagle library.

*Risk Level*: low
*Testing*: unit tests
*Docs Changes*: updated API docs
*Release Notes*: n/a

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

Mirrored from https://github.com/envoyproxy/envoy @ 21c6d13243b5d3c9decf1b24a49b72e66232953e

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

@@ -65,6 +65,9 @@ enum ProtocolType {
 
   // The Thrift proxy will use the Thrift compact protocol.
   COMPACT = 3;
+
+  // The Thrift proxy will use the Thrift "Twitter" protocol implemented by the finagle library.
+  TWITTER = 4;
 }
 
 // ThriftProtocolOptions specifies Thrift upstream protocol options. This object is used in