add livekit helper

[fbarril]

This pull request introduces a new utility module for managing LiveKit rooms via its REST API, and adds a new authentication dependency. The main focus is on providing a reusable helper class for LiveKit room operations, including secure authentication using JWT tokens.

LiveKit REST API integration:

• Added a new file src/pipecat/transports/livekit/utils.py containing the LiveKitRESTHelper class, which provides methods for managing LiveKit rooms through the REST API, including room deletion and JWT-based authentication.

Authentication dependency:

• Added the pyjwt library to the auth dependency group in pyproject.toml to support JWT token creation for LiveKit API requests.