driver: add driver for waveshare RTU relay

[flxzt]

Description

This adds a driver for Waveshare ModbusRTU Relays.
These relays speak modbus so this added driver is dependent on PR #1394 .
In my current setup the relay is used to toggle the power supply of the DUT and to toggle a jumper.
I verified that the added driver works, it has been tested with the cli, the scripting and pytest interface.

Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and description in doc/configuration.rst have been updated
• PR has been tested

[codecov]

Codecov Report

❌ Patch coverage is 60.75949% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.7%. Comparing base (e4f22e7) to head (10788a9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/modbusrtudriver.py	46.6%	24 Missing ⚠️
labgrid/driver/wavesharerturelaisdriver.py	82.1%	5 Missing ⚠️
labgrid/resource/modbusrtu.py	60.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1441     +/-   ##
========================================
- Coverage    60.7%   60.7%   -0.1%     
========================================
  Files         184     185      +1     
  Lines       15099   15156     +57     
========================================
+ Hits         9180    9211     +31     
- Misses       5919    5945     +26     

Flag	Coverage Δ
3.10	60.7% <60.7%> (-0.1%)	⬇️
3.11	60.7% <60.7%> (-0.1%)	⬇️
3.12	60.7% <60.7%> (-0.1%)	⬇️
3.13	60.7% <60.7%> (-0.1%)	⬇️
3.14	60.7% <60.7%> (-0.1%)	⬇️

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