Add support for Kubernetes arbitrary node selector

[ludelafo]

This PR implements the Kubernetes arbitrary node selector described in issue https://github.com/iterative/cml/issues/1365.

This is a work in progress and my first experience with Golang and Terraform provider development, I'm open to improvements.

Related PRs:

• https://github.com/iterative/cml/pull/1370
• https://github.com/iterative/cml.dev/pull/464