sdk: add `set_keys` method to `Client` #968

Job	Run time
Format	9s
Build no_std (nightly)	2m 41s
Build WASM (stable, -p nostr)	1m 29s
Build (stable, -p nostr)	2m 26s
Build WASM (stable, -p nostr-sdk)	2m 0s
Build (stable, -p nostr --no-default-features --features alloc)	1m 15s
Build WASM (stable, -p nostr-js)	1m 25s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	2m 9s
Build WASM (1.64.0, -p nostr)	4m 39s
Build WASM (1.64.0, -p nostr-sdk)	5m 8s
Build (stable, -p nostr --features blocking)	2m 27s
Build WASM (1.64.0, -p nostr-js)	4m 47s
Build (stable, -p nostr-sdk)	3m 43s
Build (stable, -p nostr-sdk --no-default-features)	1m 45s
Build (stable, -p nostr-sdk --features blocking)	2m 50s
Build (stable, -p nostr-ffi)	2m 41s
Build (stable, -p nostr-sdk-ffi)	2m 42s
Build (1.64.0, -p nostr)	5m 59s
Build (1.64.0, -p nostr --no-default-features --features alloc)	5m 34s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	5m 19s
Build (1.64.0, -p nostr --features blocking)	6m 59s
Build (1.64.0, -p nostr-sdk)	6m 54s
Build (1.64.0, -p nostr-sdk --no-default-features)	6m 4s
Build (1.64.0, -p nostr-sdk --features blocking)	6m 37s
Build (1.64.0, -p nostr-ffi)	8m 31s
Build (1.64.0, -p nostr-sdk-ffi)	6m 55s
	1h 43m 8s

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