feat(keyring-api): add optional Balance.metadata

[ccharly]

Adding a new metadata field for BalanceStruct. This way, asset balances can now provide more "specific" information alongside the current balance.

There is not defined shape, we leave it for the consumer and producer to be in-sync regarding the content of it.

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Note

Low Risk
Backward-compatible optional API field on balance/asset structs with added tests; no auth, security, or runtime behavior changes in this diff.

Overview
Adds an optional metadata field on BalanceStruct so producers can attach extra JSON-compatible key/value data next to amount and unit. Validation uses exactOptional(record(string(), JsonStruct)), so the key must be omitted when unused (not undefined) and values must be JSON-safe.

FungibleAssetAmountStruct now spreads BalanceStruct.schema instead of inlining unit / amount, so fungible Asset balances pick up metadata without duplicating fields. Type and runtime tests cover valid metadata on Balance and fungible assets, plus rejection of invalid shapes.

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[ccharly]

Just re-inject this schema to avoid drifting. (I almost missed it initially, but Cursor reported it, so I think this refactor makes sense to avoid that in the future)