remote/config: support drop-in files

[steelman]

Pass "dropins" variable to a template being rendered containing a list of *.yaml files in a directory named after the template being rendered with ".d" appended. For example when rendering /etc/labgrid/configuration.yaml a list of files matching /etc/labgrid/configuration.yaml.d/*.yaml will be available. Then configuration.yaml may be as simple as

{% for f in dropins %}
{% include f %}
{% endfor %}

And adding a target is possible by simply creating a file in /etc/labgrid/configuration.yaml.d directory.

[jluebbe]

Force-pushed to fix a ruff error.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.1%. Comparing base (0c01c19) to head (ae26c5d).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/config.py	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1785     +/-   ##
========================================
- Coverage    45.1%   45.1%   -0.1%     
========================================
  Files         172     172             
  Lines       13608   13612      +4     
========================================
  Hits         6144    6144             
- Misses       7464    7468      +4     

Flag	Coverage Δ
3.10	45.1% <0.0%> (-0.1%)	⬇️
3.11	45.1% <0.0%> (-0.1%)	⬇️
3.12	45.1% <0.0%> (-0.1%)	⬇️
3.13	45.1% <0.0%> (-0.1%)	⬇️
3.14	45.1% <0.0%> (-0.1%)	⬇️
3.9	45.1% <0.0%> (-0.1%)	⬇️

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[jluebbe]

Force-pushed to fix sphinx syntax.

[jluebbe]

@steelman Do you still consider this RFC? Otherwise, please remove the marker from the title, so that we can get it merged.

[steelman]

If you're fine, I am fine. Do you think this could/should be tested somehow?