SevenIO Notification Provider

[scolastico]

• I have read and understand the pull request rules.

Description

Added SevenIO notification provider
Resolves #3976

Type of change

• User interface (UI)
• 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)

[scolastico]

Reminder to myself: after this is closed, remove codespace: https://github.com/codespaces/

[CommanderStorm]

In general, this is a really well written notification provider 👍🏻

I have left a few things I noticed.
Could you add screenshots of the provider working using the following skip this part of testing (and cost as sms likely costs) for us:

• DOWN/UP
• Testing
• Cert-Expiry (see https://badssl.com/)

[CommanderStorm]

Please use HiddenInput for sensitive fields instead ^^

[scolastico]

Could you add screenshots of the provider working using the following skip this part of testing (and cost as sms likely costs) for us:

• DOWN/UP
• Testing
• Cert-Expiry (see badssl.com)

Allready tested it before opening pr:

(this is an screenshot of the debuger from SevenIO)

[CommanderStorm]

Thanks for the new notification provider!

All changes in this PR are as discussed in the contribution guide
=> moving this from v2.1 to v2.0

Merging despite #4219 (comment) being open by resolving the remaining issues myself