Add function to transform timestamp into Unix time

Teiid supports function FROM_UNIXTIME as a result of TEIID-248.

There isn't its reverse counterpart though.

This is an issue, when there is a data source which doesn't support timestamp/date/time data types:

• users choose to use long(unix time) in the source
• by using FROM_UNIXTIME function they transform the long value (unix time) into respective data type value in their virtual model
• they can read such source.
• they can't insert/update it as this would require 'INSTEAD OF TRIGGER' with ability to transform timestamp/time/date back to long value - Unix time. This part is missing.

As an example could serve MySQL's UNIX_TIMESTAMP