Add support for Whapi notification provider

[edo2313]

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Description

Added support for Whapi as a notification provider for WhatsApp. It has a generous free tier in the sandbox plan and is very simple to use. Potentially fixes #1816 as this doesn't require a business account. Also an alternative to #4316

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)

[edo2313]

Here are the screenshots.
I think I have reviewed all your changes, let me know if I missed something!

[CommanderStorm]

Thanks for the new notification provider! 🎉

All changes meet the requirements from the contribution guide
⇒ merging with junior maintainer approval

(we have changed a few formatting things => fixing these)