feat: Set MQTT publish retain flag to true

[CosminLazar]

Otherwise restarting HomeAssistant leads to the sensors having unknown values until the next scrape.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.08%. Comparing base (79ddc29) to head (a601e92).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   89.08%   89.08%           
=======================================
  Files           5        5           
  Lines         403      403           
=======================================
  Hits          359      359           
  Misses         44       44           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.