Add gtxmessaging Notification Support

[cfichtmueller]

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Description

I'd like to add a new notification provider for gtxmessaging. They provide an SMS gateway, so this is about SMS notifications.

@louislam please let me know if this is a feature you would like to have in the project. Cheers.

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

Up / Down / Certificate Expired / Test Notification

[CommanderStorm]

Notification providers do not require a discussion.
I don't know if we should classify gtxmessaging as a regional notification provider, given that their website is DE/EN or global given that they offer sending to a long country list.

When adding a notification provider, these things are common in the review:

• add all translation keys to en.json; otherwise they won't be able to be translated
• Please attach screenshots of the 3 types of events DOWN/UP, Certificate Expiry¹ and Testing
• If a field is sensitive, it should use the HiddenInput instead
• If a field is difficult to find/ an average user does not know what to put there, adding an help text is something that helps us and the users

Footnotes

1. See https://badssl.com/ for broken certs ↩

[cfichtmueller]

@CommanderStorm thank you for your reply. The PR manual explicitly states that a discussion is needed beforehand, hence my approach. May I suggest to add a small note here? Unless of course I'm the only one who misunderstood 😄

I'll go ahead and implement the provider. I'll create it as a global provider since they offer english API documentation and operate internationally.

[CommanderStorm]

I am happy with the current code.

I have changed some helptexts to be more explicit in what is being asked (f.ex. the part about the TPOA) and have added the required links.

[CommanderStorm]

Thanks for the new notification provider!

All changes in this PR are uncontroversial
⇒ merging with junior maintainer approval