[Rust][DataFusion] Implement date_trunc() function

[asfimport]

Implement the date_trunc function, as described in this PostGres manual:

https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC

 

This would allow for the following use cases to be solved:

• casting of timestamps to other resolutions, such as millis and micros

• rounding dates/timestamps 

  It also allows the user to explicitly allow the truncation of timestamp precision.

Reporter: Evan Chan / @velvia

_{Note: This issue was originally created as ARROW-12391. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/datafusion#203