[FEAT] Support for Timestamp datatype.

[xcharleslin]

Adds support for an Arrow-like Timestamp datatype in Daft. Timestamp type is parameterized by (TimeUnit, TimeZone).

Storage format

• TimeUnit and TimeZone are stored in the field only. They are stored in the Arrow format (TimeUnit enum, TimeZone Option).
• Timestamps are stored in the Arrow format: i64 representing number of TimeUnits from the UNIX epoch.

String representation

• All timestamps are printed in ISO 8601.
• The number of subsecond digits printed reflects the resolution of the timestamp type.
• All timezones are printed in offset format (+-XX:XX) regardless of the timezone.

Ingestion

• Lossless ingestion from PyArrow tables.
• Ingestion from Python datetime.datetime to Timestamp(us, tz).

Casting

• Cast to and from integer types.
• Cast to and from String type (ISO 8601 only)

Expressions

• Blackbox

Drivebys

Updated arrow_cast for Daft Physical -> Daft Logical.

• Previously, casted directly to Arrow Physical.
• Updated, casts to Arrow Logical and then to Arrow Physical.

Separate PR:

• Numeric expressions (requires Duration type)

[samster25]

Nice work!