Add Heii On-Call Notification Provider

[hevans66]

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Add notification provider for Heii On-Call.

Type of change

Please delete any options that are not relevant.

• New feature (non-breaking change which adds functionality)

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)

Setup Notification

Events

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

[hevans66]

@CommanderStorm Thanks for the review. I believe I have addressed all your concerns.

[CommanderStorm]

Okay, I changed the formatting of the code a bit to be closer to the other notification providers.
Could you review the changes I added in cc00011?

Other than that, I am happy with the current code.

I am unsure if we should change the translations in this context as suggested in #4485 (review) => I am going to leave this up to you to either accept/resolve

[hevans66]

@CommanderStorm awesome! Accepted your changes. Thanks so much for the review.

[hevans66]

@CommanderStorm let me know if there is anything else I can do to help this get merged. I'd like to work on a guide for setting up Uptime Kuma with Heii On-Call, and would like to have this in before its published.

[CommanderStorm]

Actually there is:
I missed in the previous review that this is missing the screenshot from the Testing state (the "Test"-button in above screenshot)

Other than that I would be happy to merge this into the upcoming v2.0 release.

[hevans66]

Testing screenshot has been added.

[CommanderStorm]

Thanks for the notification provider!
I think adding it is uncontroversial as it matches the code style and the appropriate tests have been performed.
⇒ merging with junior maintainer approval.