add waiting room custom page preview

[broswen]

Description

Adds a method to upload a custom waiting room page preview.
https://api.cloudflare.com/#waiting-room-create-a-custom-waiting-room-page-preview

Has your change been tested?

Yes, a test has been added to waiting_room_test.go.

Types of changes

What sort of change does your code introduce/modify?

• 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 added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[jacobbednarz]

that is great, thank you