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

**Describe the feature you'd like**

A clear and concise description of what you want to happen.
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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