exporter: Add USB hub abstraction for simpler device matching

[sjg20]

The exporter config requires full @ID_PATH strings for every USB resource, which are long and hard to maintain, particular for labs with multiple hubs.

This PR adds a 'hubs' section to the exporter config that defines USB hubs by name, base path, and a mapping of logical port numbers to USB path suffixes. Resources can then use 'hub' and 'port' in their match dict instead of a raw @ID_PATH string.

For example, instead of:

board1:
USBSerialPort:
match:
'@ID_PATH': 'pci-0000:04:00.0-usb-0:2.2.3'

the config can be written as:

hubs:
a:
base: 'pci-0000:04:00.0-usb-0:2'
ports:
7: '2.3'

board1:
USBSerialPort:
match:
hub: a
port: 7

The expansion happens at config load time before any resource matching, so it works with any USB resource type.

This feature is used to help diagnose problems - e.g. if a device's UART stops working, which USB port do I need to replug? It saves a lot of confusion mapping a string of numbers to a physical port.

This feature has been manually verified on my lab (about 30 boards) with 7 16-port hubs connected to a single machine. Operation appears to be the same before and after the change. I think modified the lab to actually use this feature and checked that things still work.

Checklist

• Documentation for the feature
• Tests for the feature
  If you add a feature other drivers/resources can benefit from:
• 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

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.3%. Comparing base (c246fab) to head (b4cc652).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/exporter.py	92.1%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1854     +/-   ##
========================================
+ Coverage    45.8%   47.3%   +1.5%     
========================================
  Files         182     182             
  Lines       14718   14756     +38     
========================================
+ Hits         6743    6991    +248     
+ Misses       7975    7765    -210     

Flag	Coverage Δ
3.10	47.3% <92.1%> (+1.5%)	⬆️
3.11	47.3% <92.1%> (+1.5%)	⬆️
3.12	47.3% <92.1%> (+1.5%)	⬆️
3.13	47.3% <92.1%> (+1.5%)	⬆️
3.14	47.3% <92.1%> (+1.5%)	⬆️

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