feat(alerting): Implement alert-level provider overrides

[TwiN]

Summary

Implements alert-level overrides

This also makes it so all providers with group overrides can now also override all provider fields, as opposed to just the files defined for the override.

Fixes #96
Fixes #919

Checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 73.93939% with 344 lines in your changes missing coverage. Please review.

Project coverage is 75.75%. Comparing base (d56a848) to head (6b9553e).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
alerting/provider/email/email.go	52.94%	26 Missing and 6 partials ⚠️
alerting/provider/gitea/gitea.go	56.06%	24 Missing and 5 partials ⚠️
alerting/provider/awsses/awsses.go	59.42%	26 Missing and 2 partials ⚠️
alerting/provider/opsgenie/opsgenie.go	68.00%	17 Missing and 7 partials ⚠️
alerting/provider/pushover/pushover.go	66.15%	14 Missing and 8 partials ⚠️
alerting/provider/custom/custom.go	77.33%	10 Missing and 7 partials ⚠️
alerting/provider/github/github.go	70.37%	13 Missing and 3 partials ⚠️
alerting/provider/twilio/twilio.go	71.69%	10 Missing and 5 partials ⚠️
alerting/provider/zulip/zulip.go	82.92%	10 Missing and 4 partials ⚠️
alerting/provider/telegram/telegram.go	78.68%	8 Missing and 5 partials ⚠️
... and 15 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #929      +/-   ##
==========================================
- Coverage   77.71%   75.75%   -1.97%     
==========================================
  Files          72       72              
  Lines        5861     6649     +788     
==========================================
+ Hits         4555     5037     +482     
- Misses       1089     1308     +219     
- Partials      217      304      +87     

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