x509-cert: introduce a `builder::AsyncBuilder` trait #1191

Job	Run time
build (1.70.0, thumbv7em-none-eabi)	26s
build (1.70.0, wasm32-unknown-unknown)	33s
test (1.70.0)	1m 6s
build (stable, thumbv7em-none-eabi)	21s
test (stable)	56s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	58s
	4m 38s

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