add support for Vultr DNS API

[cdg191]

Hi, this is a request to add support for Vultr DNS API.
Thanks.

[webprofusion-chrisc]

Thanks, we are not planning to add more providers directly to our code base (there are thousands of DNS services all with different APIs) and instead you can direct your request to the Posh-ACME project and we can integrate their provider after that.

I expect you would need to prepare the provider yourself (or contract someone to do it), unless Vultr have free accounts.

https://github.com/rmbolger/Posh-ACME

Alternatively we support a range of other providers, including scripting options, and also have our Certify DNS service for CNAME delegation of challenges.