Send notification about stage executions

[Warashi]

What this PR does:

• Add notification event group about stage executions.
• Send notification before/after stage executions.

I attached the screen capture of the Slack notification above, but this PR adds webhook notifications, too.

Why we need it:

To know the deployment progress systematically.

Which issue(s) this PR fixes:

Maybe fixes #5393

Does this PR introduce a user-facing change?: Yes

• How are users affected by this change: Users receives new notifications.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[codecov]

Codecov Report

Attention: Patch coverage is 0.81967% with 242 lines in your changes missing coverage. Please review.

Project coverage is 26.12%. Comparing base (b98a963) to head (1a2b091).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/piped/controller/scheduler.go	0.00%	82 Missing ⚠️
pkg/app/pipedv1/controller/scheduler.go	0.00%	82 Missing ⚠️
pkg/app/piped/notifier/slack.go	0.00%	39 Missing ⚠️
pkg/app/pipedv1/notifier/slack.go	0.00%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5440      +/-   ##
==========================================
- Coverage   26.24%   26.12%   -0.12%     
==========================================
  Files         452      452              
  Lines       48860    49104     +244     
==========================================
+ Hits        12821    12827       +6     
- Misses      35014    35253     +239     
+ Partials     1025     1024       -1     

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