Add `AnyWorkflow(rendering:)` to make a constant-rendering workflow

[bencochran]

Kotlin has Workflow.rendering(). Seems like a handy convenience for fakes and testing (and dependency injection where you support dynamic content but some consumers just want static renderings).

Checklist

• [X] Unit Tests
• [X] ~UI Tests~
• [X] ~Snapshot Tests (iOS only)~
• [X] I have made corresponding changes to the documentation