[SPARK-22409] Introduce function type argument in pandas_udf

## What changes were proposed in this pull request?

* Add a "function type" argument to pandas_udf.
* Add a new public enum class `PandasUdfType` in pyspark.sql.functions
* Refactor udf related code from pyspark.sql.functions to pyspark.sql.udf
* Merge "PythonUdfType" and "PythonEvalType" into a single enum class "PythonEvalType"

Example:
```
from pyspark.sql.functions import pandas_udf, PandasUDFType

pandas_udf('double', PandasUDFType.SCALAR):
def plus_one(v):
    return v + 1
```

## Design doc
https://docs.google.com/document/d/1KlLaa-xJ3oz28xlEJqXyCAHU3dwFYkFs_ixcUXrJNTc/edit

## How was this patch tested?

Added PandasUDFTests

## TODO:
* [x] Implement proper enum type for `PandasUDFType`
* [x] Update documentation
* [x] Add more tests in PandasUDFTests

Author: Li Jin <ice.xelloss@gmail.com>

Closes #19630 from icexelloss/spark-22409-pandas-udf-type.