Add support for `jiff` instead of `chrono`

[BurntSushi]

We are looking at using this crate in uv: astral-sh/uv#15925

However, in uv, we are using jiff to handle datetime. But this crate uses chrono.

Would y'all be open to a patch that lets users opt out of chrono and instead use jiff?

From a quick glance, this might not be straight-forward. For one, chrono is in your public API. And there are a number of spots where y'all are using chrono APIs internally. But I wanted to gauge if you were open to this.

[Xuanwo]

Hi @BurntSushi, thank you for raising this! Many of my friends, like @tisonkun, have great experience with jiff, so I’m open to a PR that switches from chrono to jiff.

While chrono is in our public API, we only expose it as reqsign_core::DateTime. All APIs like reqsign_core::time are intended for intra-crate use, and I don’t think many users rely on them.

As long as we keep the same interface, or deprecate it properly, I think it should be fine. I don’t recall if we expose it in any public API surfaces like ProvideCredential.

[tisonkun]

Just find some time to move core/time.rs from chrono to jiff. I can't commit to move this issue forward yet, and I can see there is still several open questions like at what level chrono types are public APIs of reqsign and how much impact would this potential breaking change would cause.

#634