sql: support timezone abbreviations in date parsing

[bobvawter]

We don't support the use of "PST" or other abbreviations when parsing timezone values. This requires access to more tzinfo fields than are readily available in the time package to resolve a candidate timezone.

If this is implemented, look at:

• https://github.com/postgres/postgres/tree/REL_10_5/src/timezone
• https://github.com/postgres/postgres/blob/REL_10_5/src/timezone/localtime.c#L1706

[bdarnell]

Note that these short abbreviations are ambiguous. For example, the abbreviation IST is used by India, Ireland, and Israel. Therefore it's not just a matter of getting more information from the tzdata files, we need a plan to handle the ambiguity.

Postgres makes this configurable: https://www.postgresql.org/docs/current/datetime-config-files.html

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