Implement ScalarValue::TimestampNanosecond -> String Support

[phillipleblanc]

Is your feature request related to a problem or challenge?

Part of #9494 to complete 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

Support converting expression that are scalar timestamp values:

Here ts is a column with the schema: Timestamp(Millisecond, None)

> describe timestamp_test;
+----------------+------------------------------+-------------+
| column_name    | data_type                    | is_nullable |
+----------------+------------------------------+-------------+
...
| time           | Timestamp(Millisecond, None) | YES         |

sql> select * from timestamp_test where ts > '2024-01-01';
External error: This feature is not implemented: Unsupported DataType: conversion: Timestamp(Nanosecond, None)

The logical plan derived from parsing the above query should be able to be turned back into SQL.

Describe alternatives you've considered

The basic pattern is:

Implement the Expr --> AST reverse code in Unparser::expr_to_sql(source link)
Add a test to expr_to_sql_ok
Note you can run the tests like

cargo test -p datafusion-sql -- expr_to_sql_ok

Additional context

No response

[goldmedal]

take