Maybe we should mark which coins are confirmed but not yet spendable in the balance?

See https://github.com/bitcoindevkit/bdk/pull/640#discussion_r912108494

The idea is that if I have 10BTC in my wallet which are confirmed, but I'll be able to spend it only in a few months, then maybe `get_balance` should reflect that, and count the coins as `locked` or something.

This would mean passing a policy to `get_balance` so that it knows with which descriptor policy we would like to spend our coins.

Since this is a pretty advanced feature, maybe we should consider adding another API (`get_balance_with_policy` or something), so that normal users can just use `get_balance` and be fine.