Update entra webhook handler for handshake

Part of uis/devops/infra/terraform/ucam-faas#16 (closed)

Large-scale refactoring of the entra webhook container to support the required changes to allow for the container to respond to Entra event-grid webhook calls.

Now creates a dedicated Flask app for webhook handling, rather than relying on the existing functions-framework app. Functions-framework is not really designed for webhook endpoints, so a dedicated Flask app makes more sense.

Updated the entra webhook config to be more in-line with other pubsub config in different repositories - relying on the google libraries to build the topic paths rather than requiring the caller to create it explicitly.

Updates testing to use a pubsub emulator rather than mock calls.