[SPARK-28088][SQL] Enhance LPAD/RPAD function
## What changes were proposed in this pull request?
This pr enhances `LPAD`/`RPAD` function to make `pad` parameter optional.
PostgreSQL, Vertica, Teradata, Oracle and DB2 support make `pad` parameter optional. MySQL, Hive and Presto does not support make `pad` parameter optional. SQL Server does not have `lapd`/`rpad` function.
**PostgreSQL**:
```
postgres=# select substr(version(), 0, 16), lpad('hi', 5), rpad('hi', 5);
substr | lpad | rpad
-----------------+-------+-------
PostgreSQL 11.3 | hi | hi
(1 row)
```
**Vertica**:
```
dbadmin=> select version(), lpad('hi', 5), rpad('hi', 5);
version | lpad | rpad
------------------------------------+-------+-------
Vertica Analytic Database v9.1.1-0 | hi | hi
(1 row)
```
**Teradata**:
**Oracle**:
**DB2**:
More details:
https://www.postgresql.org/docs/11/functions-string.html
https://docs.teradata.com/reader/kmuOwjp1zEYg98JsB8fu_A/e5w8LujIQDlVmRSww2E27A
## How was this patch tested?
unit tests
Closes #24899 from wangyum/SPARK-28088.
Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
c7f0301477
