fix(kuma-cp) use cmux for MADS server

[lobkovilya]

Summary

The existing approach of multiplexing GRPC and HTTP/1.1 on the same port doesn't work without TLS. Instead, library cmux works pretty well, that's why current PR replaces the existing approach with cmux.

Change default version of MADS API to v1alpha1 because of #1886

Full changelog

• add new library cmux
• unit tests for MADS server
• change default version of MADS API to v1alpha1

[jakubdyszkiewicz]

nice! 👏

[austince]

Thanks for this nice test!

[austince]

Should we add the same to the dp-server @lobkovilya?

kuma/pkg/dp-server/server/server.go

Lines 104 to 110 in 10936f8

	if request.ProtoMajor == 2 && strings.Contains(request.Header.Get("Content-Type"), "application/grpc") {
	d.grpcServer.ServeHTTP(writer, request)
	} else {
	// we only want to measure HTTP not GRPC requests because they can mess up metrics
	// for example ADS bi-directional stream counts as one really long request
	std.Handler("", d.promMiddleware, d.httpMux).ServeHTTP(writer, request)
	}

Or is this always run with TLS so it doesn't matter?