Add defmt support gated behind a feature flag.

[pebender]

This adds defmt support gated behind a feature flag. It adds it to the release/0.3.1 branch because (1) there are changes to the main branch that are not backward compatible, and (2) there appears to be very little work on the main range. So in order to make the defmt support most available to me and to packages that depend on 3.x (and async 0.4.x), I felt it was best made on the release/0.3.1 branch.