feat(notifications): add general notification delay

[lazou]

This PR will add the possibility to define a delay before sending a notification in the new notification system with notifications-delay. The legacy delay for emails (notification-email-delay) will still work and overwrite the general delay.

[piksel]

This does look really good. Perhaps we should even remove the email-specific delay code to reduce complexity more.
We can just put make the --notification-email-delay an alias for --notifications-delay to make it backwards compatible. That can be done in another PR though.

[codecov]

Codecov Report

Merging #1246 (3ede579) into main (f79e4b5) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1246      +/-   ##
==========================================
+ Coverage   62.70%   63.00%   +0.29%     
==========================================
  Files          23       23              
  Lines        1488     1500      +12     
==========================================
+ Hits          933      945      +12     
  Misses        471      471              
  Partials       84       84              

Impacted Files	Coverage Δ
internal/flags/flags.go	88.82% <100.00%> (+0.17%)	⬆️
pkg/notifications/notifier.go	80.00% <100.00%> (+2.64%)	⬆️

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