Add support for Slack bot token authentication

[RicardoLorenzo]

Adding Bearer token for using Slack apps instead of webhook as recommended by Slack and also used in Flux.

[stefanprodan]

@RicardoLorenzo can you please document this feature in https://github.com/fluxcd/flagger/blob/main/docs/gitbook/usage/alerting.md Thanks!

[codecov-commenter]

Codecov Report

Base: 54.68% // Head: 54.66% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (b116014) compared to base (89b0487).
Patch coverage: 58.82% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1270      +/-   ##
==========================================
- Coverage   54.68%   54.66%   -0.03%     
==========================================
  Files          81       81              
  Lines        6965     6972       +7     
==========================================
+ Hits         3809     3811       +2     
- Misses       2559     2562       +3     
- Partials      597      599       +2     

Impacted Files	Coverage Δ
pkg/notifier/client.go	29.72% <0.00%> (-1.70%)	⬇️
pkg/notifier/factory.go	0.00% <0.00%> (ø)
pkg/controller/events.go	50.80% <50.00%> (-0.44%)	⬇️
pkg/notifier/slack.go	76.47% <80.00%> (+0.71%)	⬆️
pkg/notifier/discord.go	72.22% <100.00%> (ø)
pkg/notifier/gchat.go	88.88% <100.00%> (ø)
pkg/notifier/rocket.go	75.75% <100.00%> (ø)
pkg/notifier/teams.go	76.92% <100.00%> (ø)

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[RicardoLorenzo]

@RicardoLorenzo can you please document this feature in https://github.com/fluxcd/flagger/blob/main/docs/gitbook/usage/alerting.md Thanks!

Thank you! documentation added.

[stefanprodan]

LGTM

Thanks @RicardoLorenzo 🎖️ I trust you've tested this with a Slack bot.

[RicardoLorenzo]

Thanks @stefanprodan . I do have a Slack app indeed we used for this.

For your peace of mind, this is also referenced in here. The following works which is what we are doing here.

curl https://slack.com/api/conversations.list -H "Authorization: Bearer xoxb-1234..."

Thanks again!