x509-cert: introduce a `builder::AsyncBuilder` trait #956

Job	Run time
minimal-versions	0s
build (1.75.0, thumbv7em-none-eabi)	28s
build (1.75.0, wasm32-unknown-unknown)	26s
test (1.75.0)	1m 32s
build (stable, thumbv7em-none-eabi)	23s
test (stable)	1m 24s
build (stable, wasm32-unknown-unknown)	20s
	4m 33s

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