lpad/rpad function support default value

[wangsimo0]

Feature request

StarRocks already support the rpad function and lpad function. However, the third parameter is not optional.
For easy to use purpose, it's better to make the third value optional with a default value ' ', a single blank.
That means:

1. For lpad: https://docs.starrocks.io/en-us/latest/sql-reference/sql-functions/string-functions/lpad
   It will be like:

VARCHAR lpad(VARCHAR str, INT len [, VARCHAR pad])

SELECT lpad("hi", 5);

will be equal to

SELECT lpad("hi", 5, " ");

and returns

+---------------------+
| lpad('hi', 5)       |
+---------------------+
|      hi             |
+---------------------+

2. For rpad :https://docs.starrocks.io/en-us/latest/sql-reference/sql-functions/string-functions/rpad

VARCHAR rpad(VARCHAR str, INT len [, VARCHAR pad])

SELECT rpad("hi", 5);

will be equal to

SELECT rpad("hi", 5, " ");

and returns

+---------------------+
| lpad('hi', 5) |
+---------------------+
| hi                    |
+---------------------+

[kateshaowanjou]

@predator4ann Can you make a comment here so I can assign this issue to you?

[predator4ann]

@kateshaowanjou Sure, you can assign the issue to me. Let me know if you have any further questions or concerns.