sdk: refactor `get_contact_list_public_keys` method #1070

Job	Run time
Build no_std (nightly)	1m 44s
Build FFI (stable, -p nostr-ffi)	58s
Build WASM (stable, -p nostr)	27s
Build FFI (stable, -p nostr-sdk-ffi)	1m 53s
Build (stable, -p nostr)	1m 53s
Build WASM (stable, -p nostr-sdk)	57s
Format	8s
Build (stable, -p nostr --no-default-features --features alloc)	34s
Build WASM (stable, -p nostr-js)	25s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	1m 51s
Build WASM (1.64.0, -p nostr)	5m 35s
Build (stable, -p nostr --features blocking)	2m 3s
Build WASM (1.64.0, -p nostr-sdk)	6m 9s
Build (stable, -p nostr-sdk)	3m 29s
Build WASM (1.64.0, -p nostr-js)	7m 12s
Build (stable, -p nostr-sdk --no-default-features)	1m 25s
Build (stable, -p nostr-sdk --features blocking)	2m 38s
Build (1.64.0, -p nostr)	7m 10s
Build (1.64.0, -p nostr --no-default-features --features alloc)	6m 46s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	7m 53s
Build (1.64.0, -p nostr --features blocking)	7m 16s
Build (1.64.0, -p nostr-sdk)	11m 23s
Build (1.64.0, -p nostr-sdk --no-default-features)	6m 36s
Build (1.64.0, -p nostr-sdk --features blocking)	9m 5s
	1h 35m 30s

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