Support contact_group data source

[G-Rath]

We should have a data source for contact groups; ideally it would be good if it supported selecting a contact group by it's name.

Note that StatusCake currently doesn't seem to actually check if a contact group ID is valid (e.g. is for a contact group that exists) when setting them on uptime tests, so this does have a bit of use even if it requires the ID as it can serve as a way to validate within Terraform that the contact group actually exists.