## What changes were proposed in this pull request?
This PR deals with four points as below:
- Reuse existing DDL parser APIs rather than reimplementing within PySpark
- Support DDL formatted string, `field type, field type`.
- Support case-insensitivity for parsing.
- Support nested data types as below:
**Before**
```
>>> spark.createDataFrame([[[1]]], "struct<a: struct<b: int>>").show()
...
ValueError: The strcut field string format is: 'field_name:field_type', but got: a: struct<b: int>
```
```
>>> spark.createDataFrame([[[1]]], "a: struct<b: int>").show()
...
ValueError: The strcut field string format is: 'field_name:field_type', but got: a: struct<b: int>
```
```
>>> spark.createDataFrame([[1]], "a int").show()
...
ValueError: Could not parse datatype: a int
```
**After**
```
>>> spark.createDataFrame([[[1]]], "struct<a: struct<b: int>>").show()
+---+
| a|
+---+
|[1]|
+---+
```
```
>>> spark.createDataFrame([[[1]]], "a: struct<b: int>").show()
+---+
| a|
+---+
|[1]|
+---+
```
```
>>> spark.createDataFrame([[1]], "a int").show()
+---+
| a|
+---+
| 1|
+---+
```
## How was this patch tested?
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#18590 from HyukjinKwon/deduplicate-python-ddl.
ebc124d4c4