x509-cert: introduce a `builder::AsyncBuilder` trait #907

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	25s
build (1.75.0, wasm32-unknown-unknown)	26s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	22s
test (1.75.0)	31s
test (stable)	33s
	2m 37s

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