sdk: allow to change `Keys` #967

Job	Run time
Format	7s
Build no_std (nightly)	1m 49s
Build WASM (stable, -p nostr)	1m 24s
Build WASM (stable, -p nostr-sdk)	1m 27s
Build WASM (stable, -p nostr-js)	1m 39s
Build (stable, -p nostr)	2m 3s
Build WASM (1.64.0, -p nostr)	4m 38s
Build (stable, -p nostr --no-default-features --features alloc)	1m 42s
Build WASM (1.64.0, -p nostr-sdk)	4m 55s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	1m 43s
Build WASM (1.64.0, -p nostr-js)	5m 43s
Build (stable, -p nostr --features blocking)	2m 58s
Build (stable, -p nostr-sdk)	2m 58s
Build (stable, -p nostr-sdk --no-default-features)	1m 44s
Build (stable, -p nostr-sdk --features blocking)	3m 20s
Build (stable, -p nostr-ffi)	2m 26s
Build (stable, -p nostr-sdk-ffi)	2m 38s
Build (1.64.0, -p nostr)	5m 48s
Build (1.64.0, -p nostr --no-default-features --features alloc)	5m 35s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	5m 35s
Build (1.64.0, -p nostr --features blocking)	6m 25s
Build (1.64.0, -p nostr-sdk)	7m 9s
Build (1.64.0, -p nostr-sdk --no-default-features)	6m 47s
Build (1.64.0, -p nostr-sdk --features blocking)	7m 1s
Build (1.64.0, -p nostr-ffi)	7m 14s
Build (1.64.0, -p nostr-sdk-ffi)	6m 40s
	1h 41m 28s

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