feat: refactor entra webhook container

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.