Support to uparse logical plan with timestamp cast to string

[sgrebnov]

Is your feature request related to a problem or challenge?

Part of #9494 to enhance support for Unparsing LogicalPlan to SQL String.

Converting LogicalPlans back to SQL is valuable for several usecases such as using DataFusion in federated databases (e.g. generate SQL to push down) or using DataFusion's expr APIs to programmatically create SQL.

Describe the solution you'd like

The logical plan derived from queries with time predicates, such as those involving string literals and function results cast to timestamps, should be able to be converted back into SQL. Currently, they are failing with the error Unsupported DataType: conversion: Timestamp(Nanosecond, None).

select * from taxi_trips where pickup_datetime > now()
select * from taxi_trips where pickup_datetime = '2013-08-22 06:24:12-02'

Describe alternatives you've considered

1. Implement the arrow to ast cast support for timestamp in Unparser::expr_to_sql(source link)
2. Add a test to expr_to_sql_ok

Additional context

Example issue when using DataFusion with federated database : spiceai/spiceai#1625