Add webhooks in Base class

[BAPCon]

Adds webhook functionality to the Base class, referenced in #208 :

• Create webhook
• Delete webhook
• List webhooks
• Get webhook by ID (for accessing webhook settings)
• Toggle webhook notifications
• Refresh webhook life
• Get webhook payloads

I'm struggling to find an elegant solution for generating the webhook specifications, so for now you can just pass in a dict containing said specifications.

To add:

• Iterator for webhook payloads
• Constructor of webhook specifications