feat(alerting): Add Pushover sound support

[bakito]

Summary

In extension to #405 (#129) this PR adds support to configure sound for Pushover.
https://pushover.net/api#sounds

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Added the documentation in README.md, if applicable.

[codecov-commenter]

Codecov Report

Base: 82.05% // Head: 82.06% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (e6c9a56) compared to base (277e805).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #417   +/-   ##
=======================================
  Coverage   82.05%   82.06%           
=======================================
  Files          56       56           
  Lines        4225     4226    +1     
=======================================
+ Hits         3467     3468    +1     
  Misses        590      590           
  Partials      168      168           

Impacted Files	Coverage Δ
alerting/provider/pushover/pushover.go	86.66% <100.00%> (+0.30%)	⬆️

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[TwiN]

Awesome work!
Thank you for the contribution!