feat: implement stateless udf registration

[Sl1mb0]

Closes #25

This creates a couple types to stateless-ly handle UDF registration and invocation within a query. I've opted for a draft PR as its possible things may change significantly and to leverage CI for testing.

• I've read the contributing section of the project CONTRIBUTING.md.
• Signed CLA (if not already signed).

[crepererum]

Before we merge this, can you try if that API works for monolith?

[crepererum]

That's a good test, but ON TOP of that, could we ALSO test that this works:

CREATE FUNCTION add_one()
LANGUAGE python
AS '
def add_one(x: int) -> int:
    return x + 1

def multiply_two(x: int) -> int:
    return x * 2
';


SELECT add_one(1), multiply_two(3);

(from skimming through the code I think it should)

[crepererum]

And related, could we test the behavior of this nonsense here:

CREATE FUNCTION add_one()
LANGUAGE python
AS '';

SELECT 1;

(i.e. the python code block contains NO functions)

[crepererum]

I think for the integration it would be easier if you return the physical plan here and let the caller decide if they wanna call collect or whatever.