* feat(to_unixtime): add initial implementation
* feat(to_unixtime): use Timestamp for conversion
* feat(to_unixtime): implement conversion to Result<VectorRef>
* feat(to_unixtime): make unit test pass
* feat(to_unixtime): preserve None for invalid timestamps
* feat(to_unixtime): address code review suggestions
* feat(to_unixtime): add an sqlness test
* feat(to_unixtime): adjust the assertion for the sqlness test
* Update tests/cases/standalone/common/select/dummy.sql
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Co-authored-by: Ruihang Xia <waynestxia@gmail.com>