Add optional defmt support #3562

Job	Run time
rust_msrv (ubuntu-latest)	3m 27s
timezones (ubuntu-latest, ACST-9:30)	1m 26s
timezones (ubuntu-latest, EST4)	1m 32s
rust_versions (ubuntu-latest, stable)	2m 5s
timezones (ubuntu-latest, UTC0)	1m 26s
rust_versions (ubuntu-latest, beta)	1m 11s
features_check (ubuntu-latest)	20s
timezones (ubuntu-latest, Asia/Katmandu)	1m 13s
rust_versions (ubuntu-latest, nightly)	1m 10s
timezones (macos-latest, ACST-9:30)	48s
timezones (macos-latest, EST4)	47s
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	36s
timezones (macos-latest, UTC0)	1m 0s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	15s
timezones (macos-latest, Asia/Katmandu)	57s
check-docs	22s
alternative_targets (ubuntu-latest, aarch64-linux-android)	17s
timezones (windows-latest, ACST-9:30)	3m 18s
cross-targets (x86_64-unknown-illumos)	1m 21s
timezones (windows-latest, EST4)	2m 51s
timezones (windows-latest, UTC0)	3m 19s
cross-tests (ubuntu-latest)	1m 51s
timezones (windows-latest, Asia/Katmandu)	3m 19s
no_std (ubuntu-latest, thumbv6m-none-eabi)	15s
test_wasip1 (ubuntu-latest, wasm32-wasip1)	44s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	20s
test_wasm (ubuntu-latest)	47s
	36m 57s

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