Unknown types should be marked as `unknown` and not `any`

[kylemilloy]

For TypeScript v3 or higher we should be using unknown as a type instead of any when we can't find an appropriate type to map to. You can view the TypeScript PR that introduces unknown and its purpose here.

PR incoming....

[kylemilloy]

Opened a draft for discussion. @adenh93 let me know if you have any questions or concerns and I can make changes.