Add client for fetching token from Duo Workflow Service

* This will be used by the API endpoint being added in:
  !159569
* Using RPC added in gitlab-org/duo-workflow/duo-workflow-service!12
* Ruby client here is copied over from files added in gitlab-org/duo-workflow/duo-workflow-service!29
* To test:
  * Run Gitlab via GDK
  * Run Duo Workflow Service server (`poetry run python -m duo_workflow_service.server`)
  * In gitlab rails console, run:
     `Ai::DuoWorkflow::DuoWorkflowService::Client.new(duo_workflow_service_url: "localhost:50052", current_user: User.first).generate_token`
  * The returned token can be used to authenticate requests to the Duo
    Workflow Service
  * Fixes  #471898