Workers service and registered tasks endpoint

[berggren]

Summary

Introduced the Workers service to the Go client to support retrieving and listing registered task workers from the API.

Technical Details:

• Service Implementation: Created a new WorkersService in workers.go that provides a Registered method to fetch task worker metadata from the /taskqueue/tasks/registered endpoint.
• Data Models: Defined Worker and TaskConfig structs to map API responses, including support for task-specific configurations such as labels, descriptions, and default values.
• Client Integration: Modified client.go to include the WorkersService within the main Client struct and added a Workers() accessor method for service initialization.
• Documentation: Updated README.md with a code example demonstrating how to list registered workers and access their display names and task names.
• Testing: Added workers_test.go containing unit tests for the Registered method, covering successful JSON decoding and API error handling.