* Protobuf Modernization
Remove direct usage of golang/protobuf in favor of google.golang.org/protobuf
Marshallers (protobuf and json) needed some changes to account for different APIs.
Moved to using the google.golang.org/protobuf/types/known/* for the well known types including replacing some custom Struct manipulation with whats available in the structpb well known type package.
This also updates our devtools script to install protoc-gen-go from the right location so that files it generates conform to the correct interfaces.
* Fix go-mod-tidy make target to work on all modules
This ensures that if someone does include some extension Consul does not currently make use of, that extension is actually usable. Without linking these envoy protobufs into the main binary it can't round trip the escape hatches to send them down to envoy.
Whenenver the go-control-plane library is upgraded next we just have to re-run 'make envoy-library'.
Matt Keeler