Function try_parse_custom_statement 

pub fn try_parse_custom_statement(sql: &str) -> Result<Option<ParsedStatement>>

Attempts to parse custom SQL extensions that sqlparser does not support.

Returns Ok(Some(..)) if the statement is a recognized extension, Ok(None) if it should be delegated to sqlparser, or Err on parse failure.

Supported extensions:

• CREATE MASK <name> ON <table>.<column> USING <strategy>
• DROP MASK <name>