feat(SLB-450): preview auth

[colorfield]

Package(s) involved

silverback_preview_link

Description of changes

• Checks permissions for a valid OAuth bearer token.
• Alternatively, grant access if there is a valid Preview link token for the entity

Motivation and context

To be used by the Preview app in https://github.com/AmazeeLabs/silverback-template/pull/307

How has this been tested?

Locally.