feat: expose a function for generating a new uuid

[jcrist]

Some backends have functions for returning a newly generated UUID (e.g. postgres's gen_random_uuid or duckdb's uuid). Since Ibis includes support for uuid types, it might be nice to expose this as ibis.uuid.

noted in a comment in https://github.com/ibis-project/ibis/issues/8256.

[jcrist]

cc @NickCrews - I've split your request out for a builtin ibis.uuid function here.

[jitingxu1]

/take