Provide opt out from changes to libraryDependencies

[bchallenor]

Would it be possible to provide an opt out from changes to libraryDependencies? I am shading the protobuf-java jar into my own jar and renaming its packages, so I don't want the vanilla jar added to libraryDependencies.

My preference would be for the plugin to not change libraryDependencies by default; it seems inconsistent to delegate that responsibility to the plugin given that the user is responsible for providing a protoc of a matching version. Without the libraryDependencies feature, the plugin would be independent of the protoc version. Its scope would be simply to watch .proto files and invoke the user's protoc when they change. But if backwards compatibility is a concern, it might require another setting to disable the existing behaviour.