crypto-common: add `KeyExport` trait #2690

Job	Run time
build (stable, wasm32-unknown-unknown)	23s
test (stable)	22s
build (stable, thumbv7em-none-eabi)	19s
build (1.85.0, wasm32-unknown-unknown)	21s
build (1.85.0, thumbv7em-none-eabi)	16s
test (1.85.0)	18s
	1m 59s

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