Extract tunnel code into a separate Go module

We want to extract the internal/tunnel package into a separate Go module so that it can be used by e.g. Runner and admission controllers to connect to kas.

We need to figure out:

• What is the best way to do this - either a separate repository or a submodule in this repository.
• How to generate protobuf code for the proto files. Use Buf cli maybe? We don't want to use Bazel because it'd be overkill and too complex for unfamiliar people.
• How to generate mocks