Migrate Terraform SDK to V2

[JoooostB]

Migrate to V2 of Terraform SDK and fix breaking changes as mentioned in: https://www.terraform.io/plugin/sdkv2/guides/v2-upgrade-guide.
This involves changing import paths from github.com/hashicorp/terraform-plugin-sdk to github.com/hashicorp/terraform-plugin-sdk/v2.

For the TransIP provider, the following changes in the SDK needed some attention:

• Removal of helper/mutexkv Package
  • Providers that need the functionality provided by the helper/mutexkv package are encouraged to copy the types and functions it provided into their own codebase.
• Removal of Deprecated Validation Functions
  • helper/validation.CIDRNetwork (use helper/validation.IsCIDRNetwork instead)

All of the above has been taken into account for this PR.

[aequitas]

I'm currently rebasing this onto master to have it tested and merged. Hoop to finish it this week. Sorry for the delay.