Add ipv6 option when creating a check

[duffn]

Firstly, thanks for the hard work on the library. We use it and your Terraform provider.

It would be nice to have the ipv6 option available when creating/updating a check. https://docs.pingdom.com/api/#tag/Checks/paths/~1checks/post

In turn, it would be nice to have that in your Terraform provider as well.

I can work on this if you like.