Add toolkit command for Vespa CLI with server implementations

[ldalves]

This PR introduces the toolkit command to the Vespa CLI, providing users with a suite of tools to interact with Vespa services. The implementation consists of two main server components:

1. Frontend Server: Serves the Vespa toolkit dashboard, allowing users to access and manage various Vespa functionalities - starting here with Query Builder and Query Tracer.
2. Proxy Server: Acts as a reverse proxy to Vespa services for cloud users, ensuring secure communication and handling CORS preflight requests for the frontend.

Additional changes include:

• Prompting the user for confirmation before starting the toolkit services.
• Graceful shutdown of the servers to ensure there's no abrupt termination.

This enhancement aims to provide users with a seamless interface to interact with Vespa services directly from their CLI, enhancing the overall user experience.