feat(alerting): Implement Zulip's alerts

[mehdibo]

Summary

Fixes #802

Checklist

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

[mehdibo]

Here is a preview of the message sent (the last one is the one implemented):
https://chat.zulip.org/#narrow/stream/7-test-here/topic/Gatus

[TwiN]

Great work so far!

Could you also add the new provider here:

gatus/alerting/provider/provider.go

Lines 62 to 84 in 7de5a1f

	var (
	// Validate interface implementation on compile
	_ AlertProvider = (*awsses.AlertProvider)(nil)
	_ AlertProvider = (*custom.AlertProvider)(nil)
	_ AlertProvider = (*discord.AlertProvider)(nil)
	_ AlertProvider = (*email.AlertProvider)(nil)
	_ AlertProvider = (*github.AlertProvider)(nil)
	_ AlertProvider = (*gitlab.AlertProvider)(nil)
	_ AlertProvider = (*gitea.AlertProvider)(nil)
	_ AlertProvider = (*googlechat.AlertProvider)(nil)
	_ AlertProvider = (*jetbrainsspace.AlertProvider)(nil)
	_ AlertProvider = (*matrix.AlertProvider)(nil)
	_ AlertProvider = (*mattermost.AlertProvider)(nil)
	_ AlertProvider = (*messagebird.AlertProvider)(nil)
	_ AlertProvider = (*ntfy.AlertProvider)(nil)
	_ AlertProvider = (*opsgenie.AlertProvider)(nil)
	_ AlertProvider = (*pagerduty.AlertProvider)(nil)
	_ AlertProvider = (*pushover.AlertProvider)(nil)
	_ AlertProvider = (*slack.AlertProvider)(nil)
	_ AlertProvider = (*teams.AlertProvider)(nil)
	_ AlertProvider = (*telegram.AlertProvider)(nil)
	_ AlertProvider = (*twilio.AlertProvider)(nil)
	)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 93.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (54221ef) to head (777faf6).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
alerting/provider/zulip/zulip.go	93.22%	2 Missing and 2 partials ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
+ Coverage   78.77%   78.95%   +0.18%     
==========================================
  Files          70       71       +1     
  Lines        4707     4767      +60     
==========================================
+ Hits         3708     3764      +56     
- Misses        792      794       +2     
- Partials      207      209       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[TwiN]

@mehdibo Great work!