gapic: refactor plugins to Go Protobuf v2 plugin API

Recently, the `v2` of the Go Protobuf API was released, that included improvements to the protoc plugin API. We should upgrade and refactor the Go micro-generator to use google.golang.org/protobuf/compiler/protogen ([godoc](https://pkg.go.dev/google.golang.org/protobuf/compiler/protogen)). This could significantly reduce the amount of hand-spun utility code in the Go micro-generator.

This would also probably mean significant changes to all three generators: `gengapic`, `gencli`, `gensample`.