x509-cert: introduce a `builder::AsyncBuilder` trait #252

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 30s
build (1.65.0, wasm32-unknown-unknown)	1m 22s
test (1.65.0)	1m 20s
test (stable)	27s
build (stable, thumbv7em-none-eabi)	27s
build (stable, wasm32-unknown-unknown)	28s
minimal-versions / minimal-versions	32s
	6m 6s

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