feat(pg-functions): add age(timestamp, timestamp) and age(timestamp)

[Ishaan400]

Closes #436

Adds PostgreSQL's age() to the datetime category, ported from timestamp_age / timestamptz_age in PostgreSQL's timestamp.c. Today age(...) fails with Invalid function 'age'.

• The timestamps are subtracted field by field, and negative fields borrow from the next larger one. A day borrows the length of the earlier timestamp's month, so age('2024-03-01', '2024-01-31') is 1 mon 1 day and the reverse is -1 mons -1 days.
• Accepts timestamps of any unit, with or without a time zone, dates and string literals (age('2024-03-01', '2024-01-31') works, as in PostgreSQL). Always returns Interval(MonthDayNano); other argument types are a planning error.
• age(x) is rewritten to age(current_date, x) during planning, which is how PostgreSQL defines it.
• Time zones: PostgreSQL takes both arguments' fields in the session TimeZone. Here, when an argument carries an Arrow time zone, both arguments are cast to the first such zone. A timestamp in another zone keeps its instant; a plain timestamp, date or literal is read as local time in that zone. Which argument is earlier is decided by the instants, as PostgreSQL does. This matches PostgreSQL when the session is in that zone: for example 1 day across a DST change in America/New_York, where UTC gives 23:00:00.
• Nanoseconds are kept, as in the justify_* functions.
• chrono (already a workspace dependency) becomes an optional dependency enabled by the datetime feature. functions.md marks age implemented under Date/Time; the xid form under System Information is unchanged.

Tests

• 13 unit tests in datetime/age.rs, with expected values recorded from PostgreSQL 16.15. They cover month-end and leap-day borrows, negative results, sub-second borrows, DST, mixed time zones, dates and mixed units, NULL, empty input, out-of-range timestamps, and the SQL forms (two arguments, string literals, one argument, rejected types).
• New age section in datetime.slt, with each PostgreSQL result noted.
• cargo test and cargo test --features datafusion-pg-functions/full: pass. cargo clippy --features datafusion-pg-functions/full -- -D warnings: clean. cargo fmt -- --check: clean.
• Differential check against PostgreSQL 16.15, running datafusion-postgres-cli and PostgreSQL side by side and sending the same queries through psql:
  • 27 of 28 hand-picked cases identical. The other differs only in how intervals with fractional seconds are displayed (.250000 vs PostgreSQL's .25), which justify_hours and plain interval literals also show.
  • 21,000 random timestamp pairs (1700–2260, biased toward month ends, leap days and microsecond borrows) identical.
  • 11 time-zone cases identical, using zone-tagged timestamps against PostgreSQL with the session in that zone: both DST changes in New York, month borrows in Asia/Kolkata, two different zones, zoned vs plain timestamps, dates near midnight and string literals.

Limitations

• In a session whose TimeZone is not UTC, timestamptz values reach DataFusion as UTC timestamps, so age() computes in UTC, as extract, date_trunc and ::date do today. For example, in an America/New_York session extract(day from timestamptz '2024-03-01 02:00:00+00') gives 1 here and 29 in PostgreSQL.
• age(xid) is not provided. Arrow timestamps have no infinity, so PostgreSQL's infinite cases do not arise.

[sunng87]

Awesome, thank you @Ishaan400 !