Added detail field to VAPI event object

[dragonmantank]

Description

Allows the detail field to be properly deserialized and accessed if it's part of an event.

Motivation and Context

This field now appears on some events, so should be made available. Otherwise this will just return null.

How Has This Been Tested?

Unit testing. I was not able to generate the new events using a US number so crafted a webhook response based on the API spec.

Example Output or Screenshots (if appropriate):

// If the webhook is an Event with a detail key:
$event = Factory::createFromRequest($request);
echo $event->getDetail();

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[hummusonrails]

Spent time this morning reviewing this, it looks good!