added feature to enable using without defmt

[BartMassey]

Currently this crate depends on defmt, and uses feature embassy-nrf/defmt. The dependency on heapless appears to be missing.

This PR adds the dependency on heapless, and adds a default defmt feature to this crate that depends on defmt, embassy-nrf/defmt and heapless/defmt-03.

As a result, this crate will not depend on defmt when built with --no-default-features.

I needed this to try switching from defmt to rtt-target on my crate. Unlike stock defmt, rtt-target works well with cargo-embed.

[lulf]

Thanks! Left one comment

[BartMassey]

Thank you!