feat: add Webhook handling

[Radiergummi]

What:

• Bug Fix
• New Feature
• Docs

Description:

This PR adds webhook handling to the library. This works by adding a webhook route to the application that accepts OpenAI webhook requests and dispatches an event with the request details. Users can listen to this event to implement application-specific handling.

The webhook endpoint can be customised via configuration; if that doesn't suffice, the signature verification middleware can also be used separately.

Configuration

The following new configuration parameters are available:

Parameter	Default Value	Description
openai.webhook.enabled	false	Whether webhook handling is enabled. If disabled, the route will not be registered.
openai.webhook.secret	—	The signing secret obtained from OpenAI used to verify the webhook signature.
openai.webhook.uri	/openai/webhook	URI of the webhook endpoint.
openai.webhook.domain	—	Optional domain for the route registration.
openai.webhook.middleware	'web'	Middleware(s) to use for the route registration.

Related:

This will solve #186.

[Radiergummi]

@iBotPeaches I set the client version to dev-main in the composer.json to be able to use the new webhook code already, that's why the PR is still in draft mode. Otherwise, the implementation is mostly complete; I was thinking about adding a stub for the Event Listener to make it easier to get started, but that might not even be necessary.

Also, testing was a bit more annoying than I anticipated: Since the route is only loaded if webhooks are enabled in the config file, I need to set configuration values before loading the service provider. I couldn't get it to work in Pest, so I used a PHPUnit test class instead; if you've got a better solution, I'd love to hear it 🙆‍♂️

[iBotPeaches]

Enabled CI to run and quick comment