Make BaseProvider an abstract class

[caleb-johnson]

BaseProvider was intended to be an abstract base class to be used as a reference implementation for connecting to various cloud providers (AWS, Azure, etc.), but it is implemented as a normal class with quite a few methods which throw NotImplementedErrors. It is probably better to make this an actual abstract base class.

BaseProvider is used in testing and in the client.quantum_serverless module for its ability to be instantiated with a specified ComputeResource. In order to continue making this interface available in testing, we may need something like a TestProvider class, which is duck-typed to behave like BaseProvider, even if we don't properly implement the entire interface. It would serve the purpose in testing that BaseProvider is currently used for. TestProvider should probably be an internal class used for testing, since it would likely not be able to implement the actual interface required by BaseProvider.

Open to any suggestions on implementations, but it does seem like we need a reference/abstract class here.