Add string functions support for `CHAR/VARCHAR`

[XuechunHHH]

Description

Currently in PLK-1.+, we are missing the string functions support for CHAR/VARCHAR including:

• CHAR_LENGTH
• SUBSTRING
• POSITION
• BIT_LENGTH
• OCTET_LENGTH
• TRIM_CHARS
• TRIM_LEADING
• TRIM_LEADING_CHARS
• TRIM_TRAILING
• TRIM_TRAILING_CHARS
• LIKE
• LIKE_ESCAPE

We have already supported these string functions for CHAR/VARCHAR/CLOB/STRING:

• TRIM
• UPPER
• LOWER
• CONCAT

DoD (Definition of Done)

For CHAR/VARCHAR types:

• Formalize the length/type handling behavior
• Register function signatures and implement the rest of string functions