Add support for PubSub webhook registration

[tbrenev]

WHY are these changes introduced?

We will soon have support for webhook delivery via Google Cloud PubSub.
We need to update the webhook registration function in this library to support this new delivery method.

WHAT is this pull request doing?

The register function already takes in a DeliveryMethod. This PR adds a new DeliveryMethod.PubSub,
and implements the appropriate behaviour for it. In particular, register hits the new pubSubWebhookSubscriptionCreate
and pubSubWebhookSubscriptionUpdate endpoints in the GraphQL API.

This PR also adds some documentation about registering non-HTTP-delivery webhooks (the docs for
EventBridge webhooks were missing as well).

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[tbrenev]

🎩

• Checked that pubsub webhooks are successfully registered
• Checked that eventbridge webhooks are successfully registered
• Checked that http webhooks are successfully registered, and the handler is called when the webhook fires

[thecodepixi]

This looks really good! Just left a few nit-ish comments and one question for you.