Conditionalize Avahi Service Start and Installation (#2280)

[kevinanderson1]

Ensure the Avahi daemon is installed and started only when OTBR_MDNS == "avahi". This prevents multiple mDNS stacks from running especially in container environments.

I saw in the CONTRIBUTING documentation that CI checks should pass but I'm not sure how to get them to run in my fork.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 39.77%. Comparing base (2b41187) to head (1fb1360). Report is 660 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2282       +/-   ##
===========================================
- Coverage   55.77%   39.77%   -16.01%     
===========================================
  Files          87       88        +1     
  Lines        6890     9778     +2888     
  Branches        0      721      +721     
===========================================
+ Hits         3843     3889       +46     
- Misses       3047     5691     +2644     
- Partials        0      198      +198     

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[wgtdkp]

I saw in the CONTRIBUTING documentation that CI checks should pass but I'm not sure how to get them to run in my fork.

The CI tests run in this PR. You will need to pass those failing tests.

[kevinanderson1]

@superwhd I believe this is ready for review.

[superwhd]

LGTM overall except concern of failing to include avahi in BR reference release.

[kevinanderson1]

@superwhd I've modified the PR to ensure Avahi is installed if REFERENCE_DEVICE=1. Let me know if anything else needs to be addressed.