Add Webhook functions

[n7studios]

Summary

Adds the following API functions to provide API coverage for Webhooks

• get_webhooks()
• create_webhook()
• destroy_webhook()

Testing

• testGetWebhooks(): Test that listing webhooks works.
• testCreateAndDestroyWebhook(): Test that creating and destroying a webhook works.
• testCreateAndDestroyWebhookWithEventParameter(): Test that creating and destroying a webhook with an event parameter works
• testCreateWebhookWithInvalidEvent(): Test that creating a webhook with an invalid event throws an InvalidArgumentException
• testDestroyWebhookWithInvalidRuleID(): Test that destroying a webhook that doesn't exist throws a ClientException

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)