Allow the datasource to be specified with managed alerting

[dmyerscough]

This change allows you to specify the datasource to check for managed alerts. With Grafana Cloud the default looks to be grafanacloud-prom. Whereas, with Grafana OSS its grafana.

I tested these changes on the Grafana Cloud and a private installation of Grafana.

Checklist

• The patch has appropriate test coverage
• The patch follows the style guidelines of this project
• The patch has appropriate comments, particularly in hard-to-understand areas
• The documentation was updated corresponding to the patch
• I have performed a self-review of this patch

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.35%. Comparing base (9847667) to head (7e6d114).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #217   +/-   ##
=======================================
  Coverage   92.35%   92.35%           
=======================================
  Files          27       27           
  Lines        1817     1818    +1     
=======================================
+ Hits         1678     1679    +1     
  Misses        139      139           

Flag	Coverage Δ
unittests	92.35% <100.00%> (+<0.01%)	⬆️

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[amotl]

Thank you very much, Damian. 💯

[amotl]

Hi again. We just released version 4.3.2, including your improvement. Thank you again.