Provide AT ZONE-equivalent functions where zone name is not constant

~make at_timezone it public (see @dain 's https://github.com/prestodb/presto/issues/5162#issuecomment-434127569) and add documentation~

This is required to perform AT TIME ZONE conversion when zone is not a constant

See https://github.com/prestosql/presto/issues/135#issuecomment-466559709 comment below for what should be implemented in this issue.

Ref https://github.com/prestodb/presto/issues/5162

[dain]

There are a bunch of references to this function in stackoverflow, so at the very least we would need to deprecate the old name.

[findepi]

Summing up:

what we (possibly?) need are these behaviors:

1. Make a TIMESTAMP WITH TIME ZONE from a TIMESTAMP and a timezone. function(ts::TIMESTAMP, timezone::VARCHAR) :: TIMESTAMP WITH TIME ZONE
2. Change the timezone component of a TIMESTAMP WITH TIME ZONE while preserving the instant in time. function(ts:TIMESTAMP WITH TIME ZONE, target_timezone::VARCHAR) :: TIMESTAMP WITH TIME ZONE
3. Change the timezone offset associated with a TIMESTAMP WITH TIMEZONE while preserving the local timestamp. function(ts:TIMESTAMP WITH TIME ZONE, target_timezone::VARCHAR) :: TIMESTAMP WITH TIME ZONE

• with_timezone for (1)
• at_timezone for (2)
• currently no function for (3)