dsa: gate signing under `hazmat` feature (#859) #816

Job	Run time
build (thumbv7em-none-eabi, 1.81.0)	14s
test (1.81.0)	17s
build (thumbv7em-none-eabi, stable)	22s
test (stable)	18s
build (wasm32-unknown-unknown, 1.81.0)	13s
build (wasm32-unknown-unknown, stable)	17s
	1m 41s

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