Support custom_objects

[raymondpressly]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
Client methods to fetch and search custom objects in Zendesk.

Describe alternatives you've considered

• Forking the repository and adding our own functionality
• Writing a separate client locally

Additional context
https://developer.zendesk.com/api-reference/custom-data/custom-objects/custom_object_records

[PierreBtz]

@raymondpressly looks like a good idea. PR welcomed!