feat: provide a hermetic LabGrid coordinator for `rules_labgrid`

This exposes a py_library available under @rules_labgrid//bazel/labgrid/mock. This library provides a Python @contextmanager, that can be used to set up a Labgrid coordinator running on the local machine.

Example:

py_library(
    # ...
    deps = ["@rules_labgrid//bazel/labgrid/mock"]
)

from bazel.labgrid.mock import coordinator

# ...

with coordinator() as c:
    # Do something here with `c.url`, `c.tags` and `c.place`

We currently don't support running SSH commands. This will be done in a future MR.