feat: make the local harness work standalone

the local harness was only usable as a context manager.
this commit adds a new target:

`bazelisk run //bazel/labgrid/local:harness`

this makes the harness available as a standalone program, which sets
up a dummy place and tags for it to be used by the labgrid client.

the harness chooses a free port on the machine and outputs its url
to stdout.

the program is terminated  using keyboard interrupt.