x509-cert: introduce a `builder::AsyncBuilder` trait #432

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 33s
build (1.65.0, wasm32-unknown-unknown)	1m 10s
test (1.65.0)	1m 10s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	22s
build (stable, wasm32-unknown-unknown)	16s
minimal-versions / minimal-versions	25s
	5m 16s

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