Support defmt framework

[therealfrauholle]

We are using this crate heavily in an embedded context. The defmt framework is a well-known effort by ferrous systems to provide efficient logging and debugging for embedded devices. We wish to use uom together with this framework.

Arbitrary types can be logged through this framework, though they need to implement defmt::Format. So far we had to wrap uom values in a structure in order to log them, but I think it makes sense to provide this upstream. The new functionality should probably be feature gated similar to the serde functionality.

I will make a pull request for a proposal soon.

[iliekturtles]

Sorry for the delays in catching up on these issues/PRs. PR for this feature is welcome. Let's discuss MSRV issues in #216.

[iliekturtles]

MSRV bumped to 1.60.0 in #422.

[asasine]

With MSRV at 1.65.0 as of 313bf68, is this possible now?