feat: extract `runner` library

The //labgrid/run CLI now uses a new library //bazel/labgrid/runner. This will simplify the creation of custom runners to be used with workflow rules such as labgrid_genrule, labgrid_run_binary and labgrid_test.

I had to do a lot of refactoring, so I'd recommend reviewing this commit-by-commit. I have focused on introducing the right abstraction, so the implementation of runner can still use a bit of refactoring. I've raised Refactor runner (#47 - closed) as a follow-up.

Closes #45 (closed).