Support secrets for notification_url

[jlaska]

Since the NOTIFICATION_URL contains app and user defined tokens, this PR adds support for the parameter to be loaded from a secret file. Relevant documentation updated to note that a file path is supported.

[piksel]

This used to not be possible to do due to a bug in cobra/viper/pflags. It should have been fixed now, so this might work as expected. Have you tried it with multiple values etc.?

[piksel]

No, the problem is still there. The StringArray flags only appends the values passed to Set, which means that additionally to the file contents, there will always be the file name (which probably isn't a valid notification URL).
There is a way to update this correctly though, I will make a new PR for this amend the PR.

[codecov]

Codecov Report

Merging #1300 (88d7277) into main (e983beb) will increase coverage by 0.58%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##             main    #1300      +/-   ##
==========================================
+ Coverage   63.65%   64.23%   +0.58%     
==========================================
  Files          23       23              
  Lines        1527     1552      +25     
==========================================
+ Hits          972      997      +25     
  Misses        465      465              
  Partials       90       90              

Impacted Files	Coverage Δ
internal/flags/flags.go	89.60% <89.47%> (+0.48%)	⬆️
pkg/notifications/slack.go	95.12% <0.00%> (+0.25%)	⬆️
pkg/filters/filters.go	83.63% <0.00%> (+2.38%)	⬆️

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[jlaska]

This is great, thank you @piksel !