`signature`: add `MultipartSigner` and `MultipartVerifier` #2584

Job	Run time
test (stable)	10s
no_std (1.85.0, thumbv7em-none-eabi)	17s
no_std (1.85.0, wasm32-unknown-unknown)	18s
no_std (stable, thumbv7em-none-eabi)	18s
test (1.85.0)	20s
no_std (stable, wasm32-unknown-unknown)	8s
minimal-versions	0s
	1m 31s

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