Added TIMESTAMP_NTZ type support with precision to Snowflake dialect

[romanoff]

Adding support for optional precision for TIMESTAMP_NTZ type in snowflake. Example: TIMESTAMP_NTZ(9)
It's supported in spec: https://docs.snowflake.com/en/sql-reference/data-types-datetime
Specifically

All timestamp variations, as well as the TIMESTAMP alias, support an optional precision parameter for
fractional seconds (for example, TIMESTAMP(3)). Timestamp precision can range from 0 (seconds) 
to 9 (nanoseconds). The default precision is 9.

Example of sql:

SELECT CAST(TO_CHAR(CURRENT_DATE, 'YYYY-MM-DD')||' 01:00:00' AS TIMESTAMP_NTZ(9))

[iffyio]

LGTM! Thanks @romanoff!