Added webhooks and tests for messages api

[SecondeJK]

This PR adds support for Webhooks in the Messages Client.

Description

There are six possible channels that can used to hydrate a webhook. The webhook is only one per channel, not for every available type as there would be too many classes. The webhook classes act as DTOs, so have fromArray() and toArray() which wraps a data property only with a magic getter.

The webhooks have no magic setter and are marked as final because they should be immutable.

Motivation and Context

Missing feature.

How Has This Been Tested?

Hydration tests have been added.

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.