add a PKCS 12 builder to pkcs12 crate #2464

Job	Run time
build (stable, wasm32-unknown-unknown)	16s
test (1.85.0)	30s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	28s
build (1.85.0, wasm32-unknown-unknown)	20s
build (1.85.0, thumbv7em-none-eabi)	16s
minimal-versions / minimal-versions	28s
	2m 38s

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