[V1] Complete datetime and interval types

[RCHowell]

Description

PartiQL does not implement the SQL datetime operators; here's what needs to be done: see SQL-99 4.7.3 p.29.

Tasks

• [DRAFT] Defines spi.datetime classes #1656
• Add INTERVAL_YM and INTERVAL_DT to PTYPE
• datetime - datetime -> interval
• datetime + interval -> datetime
• datetime - interval -> datetime
• interval + datetime -> datetime
• interval + interval -> interval
• interval - interval -> interval
• interval * numeric -> interval
• interval / numeric -> interval
• numeric * interval -> interval

[RCHowell]

Awaiting PR feedback on additions to Datum before fixing the build hence still draft. Fixing the builtin functions will be a lot of work which is subject to change based upon the modeling.

[RCHowell]

need rebase on #1658

[RCHowell]

After that, I'll address the build. Working on plans now.