`signature`: add `MultipartSigner` and `MultipartVerifier` #2578

Job	Run time
no_std (1.85.0, wasm32-unknown-unknown)	20s
test (1.85.0)	15s
test (stable)	25s
no_std (1.85.0, thumbv7em-none-eabi)	14s
no_std (stable, thumbv7em-none-eabi)	20s
no_std (stable, wasm32-unknown-unknown)	10s
minimal-versions	0s
	1m 44s

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