fmt: add APIs for skipping weekday checking

[BurntSushi]

In the course of doing some experiments with using Jiff in gix-date, I
discovered that it currently relies on the fact that parsing RFC 2822
datetimes (and similar) does not do error checking if the date and
weekday are inconsistent. Prior to this PR, Jiff required that the
weekday and date were consistent. This PR adds APIs for skipping this
check.

For RFC 2822, this adds a new relaxed_weekday configuration on the
DateTimeParser builder.

For the strtime APIs, we achieve this by adding more granular accessors
and mutators on BrokenDownTime. So now folks can do this:

let tm = BrokenDownTime::parse("%a, %F", "Wed, 2024-07-27")?;
tm.set_weekday(None);
assert_eq!(tm.to_date().unwrap(), jiff::civil::date(2024, 7, 27));

Where the above succeeds even though 2024-07-27 was a Saturday. It's a
little verbose, but my hope is that this is a rarely needed thing.

This PR also adds getters and setters for individual fields on a
jiff::fmt::strtime::BrokenDownTime. This affords a bit more flexibility
in terms of what can be parsed and formatted. For example, it
lets one parse just a weekday:

use jiff::{civil::Weekday, fmt::strtime::BrokenDownTime};

let tm = BrokenDownTime::parse("%a", "Sat").unwrap();
assert_eq!(tm.weekday(), Some(Weekday::Saturday));