support `format` argument for `date`, `datediff`, `timestamp`, `unix_timestamp`, etc

**Describe the feature you'd like**

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Currently we only support the following fixed three formats:
- yyyy-mm-dd
- yyyymmdd
- yyyy-mm-dd hh:mm:ss

We may allow user to provide a `format` argument to these functions.

**Additional context**

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Look for the date, datediff, timestamp, unix_timestamp function implementations in the codebase, likely in SQL function modules. Understand the current fixed format parsing and output logic. The task is to extend these functions to accept a format argument, requiring changes to function signatures, parsing, and possibly validation. Check existing tests for these functions to see how they are currently tested and add new tests for the format argument.

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