feat: add notification provider SendGrid

[sctnightcore]

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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)

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[CommanderStorm]

Overall, this mostly looks good.
I have left a few requests for changes.

[CommanderStorm]

From the code side, this would be a merge. I missed asking you for the screenshots in the description.
We require screenshots to ensure that there are no glaring bugs and because testing notification providers do not scale.

=> Could you send a test/up/down or cert-expiry notification via this monitor post a screenshot of it?

[sctnightcore]

From the code side, this would be a merge. I missed asking you for the screenshots in the description. We require screenshots to ensure that there are no glaring bugs and because testing notification providers do not scale.

=> Could you send a test/up/down or cert-expiry notification via this monitor post a screenshot of it?

I've added a photo to the description.
Thank you for the review.

[CommanderStorm]

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}