Adds OIDC token authentication to the webhook endpoint. Entra Webhook
container must be configured with:
- Tenancy id, used to verify `tid` in claim and generate token issuer
  URL.
- Entra application id, used to verify the `aud` in claim.
- Entra expected principal app id, used to verify the `azp` in claim.
- Entra expected principal object id, used to verify the `sub` and `oid`
  in claim.

Webhook handler was enforcing that the webhook data payload was JSON
before publishing to the downstream PubSub topic. This removes this
functionality as it is not required.

The entrypoint function for running the application is excluded from
coverage statistics, as it only starts the server and has no
functionality beyond that, making it unuseful to test.

Add authentication to webhook handler endpoint

See merge request !43