API key support for Terraform provider

Currently the only automated way to create API keys is via the GraphQL API. While this can be made to work through custom scripting, if the remainder of the Spacelift configuration is done with terraform, it creates a disparity in overall system configuration.

The ability to configure API keys via terraform (both secret and OIDC) would allow for better consistently for teams who have strict requirements around manual changes to production deployment tooling and make it easier to implement long term sustainability with IAC around API tokens.