Can we add udtf to FunctionRegistry?

[niebayes]

The FunctionRegistry provides interfaces for accessing user defined functions, aggregate functions, window functions ,etc.
But it lacks of an interface for accessing registered table functions.

Can we add a udtf method to FunctionRegistry?

[alamb]

@niebayes it seems like a reasonable plan to me! Let's make this an issue!

[Jefffrey]

Feature will be tracked on #15095