Webhook Endpoints: Add Deliveries Methods - #164

Summary

Adds methods for the webhook delivery history endpoints:

• get_webhook_endpoint_deliveries(): List a webhook endpoint's deliveries from the last 30 days, filterable by status and event type.
• get_webhook_endpoint_delivery(): Get a single delivery with its attempts, and optionally the events sent.

Testing

• testGetWebhookEndpointDeliveries: Test deliveries are returned for a webhook endpoint.
• testGetWebhookEndpointDeliveriesWithStatus: Test the status filter.
• testGetWebhookEndpointDeliveriesWithInvalidID: Test an invalid webhook endpoint ID returns an error.
• testGetWebhookEndpointDelivery: Test a delivery triggered by creating a subscriber is returned, with the event type filter and payload included when requested.
• testGetWebhookEndpointDeliveryWithInvalidID: Test an invalid delivery ID returns an error.

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)