[SPARK-18777][PYTHON][SQL] Return UDF from udf.register

## What changes were proposed in this pull request?

- Move udf wrapping code from `functions.udf` to `functions.UserDefinedFunction`.
- Return wrapped udf from `catalog.registerFunction` and dependent methods.
- Update docstrings in `catalog.registerFunction` and `SQLContext.registerFunction`.
- Unit tests.

## How was this patch tested?

- Existing unit tests and docstests.
- Additional tests covering new feature.

Author: zero323 <zero323@users.noreply.github.com>

Closes #17831 from zero323/SPARK-18777.