util/agentwrapper: Implement _labgrid-agent

[AiyionPrime]

a helper to execute agents on the exporter host.

Description

This hasn't been tested yet, but is a mere discussion point.

The thesis: On a host where labgrid has been installed and the exporter is supposed to run
we already have all the dependencies installed (at least in an environment, not necessarily an isolated one).

This means it should not only not be necessary to copy over the agent file, as its already present on the exporter host,
but not necessary to install dependencies globally on it either.

This introduces a new command _labgrid-agent, which due to the leading underscore should not confuse users using autocompletion.

Considerations

The approach has a considered drawback:
Currently the client uses the agentwrapper and the agents/ shipped with the client matching its version perfectly.
With this change it would only use the latter and potential upcoming incompatibilities in the wrapper could show up, in case somebody installed different versions of labgrid on the client- and the exporterhost.

I think the risk is low though, as the wrapper does not change much anymore and a version check could be introduced, once its behaviour is changed in a breaking way.

Checklist

• Documentation for the feature
• Tests for the feature

• The arguments and description in doc/configuration.rst have been updated

• Add a section on how to use the feature to doc/usage.rst

• Add a section on how to use the feature to doc/development.rst

• PR has been tested

• Man pages have been regenerated

Fixes #1790