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

[iBotPeaches]

  Problem 1
    - Root composer.json requires symfony/psr-http-message-bridge ^8.0.0 -> satisfiable by symfony/psr-http-message-bridge[v8.0.0-BETA1, ..., 8.1.x-dev].
    - symfony/psr-http-message-bridge[v8.0.0-BETA1, ..., 8.1.x-dev] require php >=8.4 -> your php version (8.2.29) does not satisfy that requirement.

Looking back at this, looks like v8 of psr bridge is PHP 8.4 or above. Our min of 8.2 can hopefully be met with an older version of that. I normally cut client/laravel together, but I see its a chicken/egg problem here with the dependency. I'll cut a client release this week.

[iBotPeaches]

v0.19.0 of client is out. This should be workable