x509-cert: introduce a `builder::AsyncBuilder` trait #840

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 21s
test (1.65.0)	1m 26s
build (1.65.0, wasm32-unknown-unknown)	1m 22s
test (stable)	26s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	23s
minimal-versions / minimal-versions	32s
	5m 53s

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