corepc-types: `ListUnspentItem`'s `amount` should be an `Amount` instead of `SignedAmount`

[storopoli]

The amount field in ListUnspentItem (returned by the listunspent RPC) is typed as SignedAmount, but UTXO amounts are always non-negative per the Bitcoin protocol.

corepc/types/src/model/wallet.rs

Lines 684 to 686 in 87c72e7

	/// The transaction amount.
	#[serde(default, with = "bitcoin::amount::serde::as_btc")]
	pub amount: SignedAmount,

Bitcoin Core's own RPC documentation does not indicate that this field can be signed/negative.
See https://bitcoincore.org/en/doc/29.0.0/rpc/wallet/listunspent/:

In contrast, signed amounts are appropriate for other RPCs (e.g., listtransactions) where negative values indicate outflows/sends/fees.
See https://bitcoincore.org/en/doc/29.0.0/rpc/wallet/listtransactions/:

[tcharding]

Sounds legit, lets fix it. Thanks man.