Implement GET /cvds/{group}/{name} endpoint

[SuperStrongDinosaur]

Implement the GET /cvds/{group}/{name} endpoint in the host orchestrator to allow retrieving a specific CVD instance by its group and name.

This is achieved by:

• Extending ListCVDsActionOpts and ListCVDsAction to support filtering by CVD name.
• Updating listCVDsHandler.Handle to extract the name variable from the request and pass it to ListCVDsActionOpts.
• Registering the GET /cvds/{group}/{name} route in Controller.AddRoutes using listCVDsHandler.
• Adding unit tests in listcvdsaction_test.go to verify the filtering logic and error handling (404 when not found).
• Adding a routing test in controller_test.go to ensure the new endpoint is correctly registered.

This change preserves the original behavior of GET /cvds and GET /cvds/{group}.