Maintenance: validate domain strings for all providers

[qdm12]

All domain strings are not validated for now. It would be good to check at a global scope the domain is

• empty
• a valid fqdn (without trail dot)

Some providers only use a host and the domain json value can be the empty string (constant value in the code), for example duckdns the domain is always duckdns.org. goip.de can have the domain empty, which defaults to goip.de, but it can also be set for example to goip.it.

[CyberAustin]

I'm taking a stab at this right now. Mostly got it working, just need to get the code to my dev machine at home so I can run all the linting and formatting.

[qdm12]

Awesome (no pressure either)
You can probably develop with the development container (see .devcontainer directory) if you have Docker and VSCode installed, that's what I use (from a directory in WSL2 on windows)

[CyberAustin]

Yeah, I use your include dev container. It's just that I'm at work and only have access to limited resources during work hours.

[CyberAustin]

PR created: #638