Fga type updates

[tribble]

Description

Update FGA to match our conventions:

• Use duck types when possible
• Use TypedDict for complex input types
• Use string literals rather than enums
• Re-export input types from the types module for convenience

Stacked on #335

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.