ffi: add `set` and `get` custom field metadata #1045

Job	Run time
Build FFI (stable, -p nostr-ffi)	1m 5s
Build no_std (nightly)	1m 56s
Build WASM (stable, -p nostr)	24s
Build FFI (stable, -p nostr-sdk-ffi)	1m 53s
Build (stable, -p nostr)	2m 33s
Build WASM (stable, -p nostr-sdk)	51s
Build (stable, -p nostr --no-default-features --features alloc)	24s
Build WASM (stable, -p nostr-js)	33s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	1m 58s
Format	5s
Build WASM (1.64.0, -p nostr)	6m 5s
Build (stable, -p nostr --features blocking)	2m 43s
Build WASM (1.64.0, -p nostr-sdk)	5m 44s
Build (stable, -p nostr-sdk)	4m 0s
Build (stable, -p nostr-sdk --no-default-features)	1m 45s
Build WASM (1.64.0, -p nostr-js)	5m 58s
Build (stable, -p nostr-sdk --features blocking)	2m 34s
Build (1.64.0, -p nostr)	7m 4s
Build (1.64.0, -p nostr --no-default-features --features alloc)	6m 28s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	6m 18s
Build (1.64.0, -p nostr --features blocking)	6m 49s
Build (1.64.0, -p nostr-sdk)	9m 30s
Build (1.64.0, -p nostr-sdk --no-default-features)	6m 32s
Build (1.64.0, -p nostr-sdk --features blocking)	9m 19s
	1h 32m 31s

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