Add API key placeholder to NLS providers

[martinfleis]

535 updated urls of NLS providers but since that now requires an API key, we need to add a placeholder for it, like with any other resource requiring a registration.

[pushing-boulders]

Thanks for having this PR @martinfleis. I was working on some updates in TileProviders.jl and was running into an issue because of this.

To the maintainers, any chance of getting this PR merged soon?

[brunob]

bump @jieter ?

[pushing-boulders]

Thank you!