Support for adding table functions to schemas

### Is your feature request related to a problem or challenge?

Table functions are currently registered globally, but it'd be great to support namespacing them within schemas, like any other tables.

I’d be happy to take this on if it’s a feature that’s likely to be accepted, it seems to be a purely additive change.

If you’re open to it, I’d appreciate some input on the best way to approach the implementation and what aspects I should consider.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points to begin with. First clarify the intended schema namespacing behavior and implementation approach with maintainers; done means table functions can be registered and resolved within schemas without breaking existing global registration.

Written by the indexing model from the issue text.