add USBHub resource and export

[jluebbe]

This resource is a bit unusual, as there is no corresponding driver, as it is only useful to monitor whether the expected USB hubs are detected by an exporter.

In larger labs, there are often USB hubs that are considered part of the infrastructure. When these fail (either temporarily or permanently), the connected devices aren't detected any more. For users of individual places, this leads to unavailable resources without a clear cause.

By configuring USBHub resources for the infrastructure hubs in the exporters, we can start monitoring hub availability centrally via the coordinator.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 56.5%. Comparing base (b90ac44) to head (8e4ce3d).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/resource/udev.py	57.1%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1548   +/-   ##
======================================
  Coverage    56.5%   56.5%           
======================================
  Files         168     168           
  Lines       13116   13124    +8     
======================================
+ Hits         7417    7422    +5     
- Misses       5699    5702    +3     

Flag	Coverage Δ
3.10	56.5% <62.5%> (+<0.1%)	⬆️
3.11	56.5% <62.5%> (+<0.1%)	⬆️
3.12	56.5% <62.5%> (+<0.1%)	⬆️
3.13	56.5% <62.5%> (+<0.1%)	⬆️
3.9	56.5% <62.5%> (+<0.1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.