x509-cert: introduce a `builder::AsyncBuilder` trait #565

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	24s
build (1.75.0, wasm32-unknown-unknown)	24s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	25s
test (1.75.0)	23s
test (stable)	23s
	2m 16s

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