Unify granularity / field-name parsing across temporal functions

[sdf-jkl]

Is your feature request related to a problem or challenge?

Working on #18319 I noticed that date_trunc defines its own DateTruncGranularity enum.

datafusion/datafusion/functions/src/datetime/date_trunc.rs

Lines 56 to 67 in 2c7af17

	enum DateTruncGranularity {
	Microsecond,
	Millisecond,
	Second,
	Minute,
	Hour,
	Day,
	Week,
	Month,
	Quarter,
	Year,
	}

date_part solves the same parsing problem differently — it delegates to arrow_cast::parse::IntervalUnit::from_str:

datafusion/datafusion/functions/src/datetime/date_part.rs

Lines 230 to 244 in 2c7af17

	IntervalUnit::Microsecond => seconds_as_i32(array.as_ref(), Microsecond)?,
	IntervalUnit::Nanosecond => seconds_ns(array.as_ref())?,
	// century and decade are not supported by `DatePart`, although they are supported in postgres
	_ => return exec_err!("Date part '{part}' not supported"),
	}
	} else {
	// special cases that can be extracted (in postgres) but are not interval units
	match part_trim.to_lowercase().as_str() {
	"isoyear" => date_part(array.as_ref(), DatePart::YearISO)?,
	"qtr" | "quarter" => date_part(array.as_ref(), DatePart::Quarter)?,
	"doy" => date_part(array.as_ref(), DatePart::DayOfYear)?,
	"dow" => date_part(array.as_ref(), DatePart::DayOfWeekSunday0)?,
	"isodow" => date_part(array.as_ref(), DatePart::DayOfWeekMonday0)?,
	"epoch" => epoch(array.as_ref())?,
	_ => return exec_err!("Date part '{part}' not supported"),

This leads to a user-visible inconsistency between the two functions. For example, date_part accepts "mon" | "mons" | "month" | "months", while date_trunc only accepts "month".

IntervalUnit, however, is the wrong enum for either of these functions — it exists to parse Postgres INTERVAL literals (INTERVAL '1 year 2 months'), which is why it has Century / Decade but no Quarter (Postgres rejects INTERVAL '1 QUARTER'). The right counterpart is arrow_arith::temporal::DatePart, which represents the date-part field namespace, includes Quarter, and is #[non_exhaustive]:
https://github.com/apache/arrow-rs/blob/8091f3f17b2de355f7c47e7a0907000d308f8f3e/arrow-arith/src/temporal.rs#L38-L79

Describe the solution you'd like

I propose:

• Add a FromStr impl for arrow_arith::DatePart (with the Postgres alias map currently inside IntervalUnit::from_str plus the date-part-only field names).
• Migrate date_part, date_trunc, and the Spark variants (date_trunc, trunc, time_trunc, date_part) to the new parser. The unparser sites in sql/src/unparser/utils.rs can also use it as an input canonicalizer.

Describe alternatives you've considered

No response

Additional context

No response