Add OneSender Webhook notification

[hasanbasri1993]

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Description

Fixes #(issue)

Type of change

Please delete any options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• User interface (UI)
• New feature (non-breaking change which adds functionality)
• Breaking change (a fix or feature that would cause existing functionality to not work as expected)
• Other
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

Event	Before	After
UP	-
DOWN	-
Certificate-expiry	-
Testing	-

[CommanderStorm]

Looks mostly good.

Could you please attach the screenshots we request in the contribution guide?
I have attached a template to the PR-Description.
The reason why we require this is that verififying that these notification providers is quite time-consuming for us and somewhat simple for you (as you already tested it).

[CommanderStorm]

(There are a few changes that I am going to do, but those are "editorial comments")

Thanks for the new notification provider! 🎉

Note

This PR is part of the v2.0 merge window => see #4500 for the bugs that need to be addressed before we can ship this release ^^

_{All changes in this PR are small and uncontroversial ⇒ merging with junior maintainer approval}