Support setting fallback origin on a zone

[Evesy]

Supports getting/setting the fallback_origin on Cloudflare Zones

Description

As part of Cloudflare's SSL for SaaS offering, a default origin server (fallback_origin) can be configured on any enabled zones.

Ultimately I'd like to be able to set this configuration option via a Terraform resource however I'm unable to do so until the Go client supports this endpoint.

The API endpoint is undocumented in the API docs but details can be found here

Has your change been tested?

• Automated tests added
• Manually tested getting the fallback origin for a zone (api.FallbackOrigin("<ZONEID>")) & updating the fallback origin for a zone (api.UpdateFallbackOrigin("<ZONEID>", newOrigin))

Screenshots (if appropriate):

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.

[jacobbednarz]

LGTM, thanks!

[patryk]

Looks good, thanks @Evesy !