Add CloudEvents endpoint validation to webhook handler proxy

Description

MS Graph API partner event topics can only be subscribed to using CloudEvents - which uses a different endpoint validation scheme than we currently support (which is the MS Event Grid scheme endpoint validation).

We need to support Graph API partner topics - so need to add this endpoint validation support to the proxy.

Further details

Find documentation for CloudEvents https://learn.microsoft.com/en-us/azure/event-grid/security-authentication#endpoint-validation-with-cloudevents-v10

The webhook handler proxy should support both validation schemes without needing to be configured via an environment variable or similar.

Task list

• Add alternate CloudEvents endpoint validation
• Deploy to dev environment
• Test using Azure sandbox subscription by creating a custom topic and subscribing to it using the CloudEvents delivery schema

Acceptance criteria

• Webhook endpoint can handle CloudEvents or Event Grid schema webhook endpoint validation

Links/references

https://github.com/cloudevents/spec/blob/v1.0/http-webhook.md#4-abuse-protection