Add Infinity check functions

[firebladed]

Added ability to check if interval defined as +/- infinity

Adds two functions

• is_infinite()
• is_positive_infinity()

based on descriptions in https://www.postgresql.org/message-id/E1r2rB1-005PHm-UL%40gemulon.postgresql.org

• is_negative_infinity() is possible addition if beneficial

Am new to rust so please point out any improvements

Does your PR solve an issue?

No
can create one is useful for tracking, but as could not find one and looks simple enough to fix directly

Is this a breaking change?

No, Do not believe so

[abonander]

Can you add cases to the integration test as well?

sqlx/tests/postgres/types.rs

Lines 642 to 668 in f8afe99

	test_prepared_type!(interval<PgInterval>(
	Postgres,
	"INTERVAL '1h'"
	== PgInterval {
	months: 0,
	days: 0,
	microseconds: 3_600_000_000
	},
	"INTERVAL '-1 hours'"
	== PgInterval {
	months: 0,
	days: 0,
	microseconds: -3_600_000_000
	},
	"INTERVAL '3 months 12 days 1h 15 minutes 10 second '"
	== PgInterval {
	months: 3,
	days: 12,
	microseconds: (3_600 + 15 * 60 + 10) * 1_000_000
	},
	"INTERVAL '03:10:20.116100'"
	== PgInterval {
	months: 0,
	days: 0,
	microseconds: (3 * 3_600 + 10 * 60 + 20) * 1_000_000 + 116100
	},
	));

[firebladed]

added infinity and -infinity value tests, but not sure how to account for postgress sql version (>= 17)