equinix_metal_connection should support API field contact_email

[ocobles]

Trying to create a connection using a project based API token I got "422 Contact email can't be blank"

The contact email field is always required, it's just that when you use a personal API key, contact email is inferred based on the user making the request. The equinix_metal_connection resource should let specify a contact_email argument and add Note in the documentation to explain the limitation depending on the user/project token