Fix gitlab_deploy_token.username during resource creation

Currently, when creating a new gitlab_deploy_token resource with the (optional) username attribute not specified, then the resultant state's username attribute value is null.

This is despite the fact that the GitLab APIs POST /projects/:id/deploy_tokens (docs) and POST /groups/:id/deploy_tokens (docs) do indeed return a (generated) value for the username attribute.

Please see https://gitlab.com/theipster/terraform-provider-gitlab/-/pipelines/884877003 for the failing test that replicates the issue.

This merge request fixes the create method by populating the username attribute using the value from the API response.