`TxBuilder`: Add an option to only use confirmed utxo's

[ErikDeSmedt]

Describe the enhancement

I want to create a transaction and ensure that prevout has been confirmed onchain.

Similar to ChangeSpendPolicy I'd like to introduce a ConfirmationSpendPolicy.
This could be used to spend only Confirmed, Trusted or Any output.

The code to use it could look like the example below

wallet.create_tx()
    .add_recipient(address.script_pubkey(), payment_amount)
    .only_spend_confirmed_utxos()
    .finalize()

Use case

When creating a transaction I want to ensure it cannot be doublespent.
If I use an unconfirmed and untrusted prevout, I cannot guarantee the transaction will not be doublespent

Additional context

[ValuedMammal]

As an alternative to the enum idea, I think we could easily represent the confirmation policy as a BTreeMap<KeychainKind, u32> where an entry defines the number of confirmations needed to spend an output on that keychain. The API could look like this

/// The inputs to the transaction must be confirmed...
pub fn must_use_confirmed(&mut self) -> &mut Self {
    // ensure that each value in the map is at least 1
    // ...
}

/// Minimum number `n` of confirmations required to spend a UTXO of the given `keychain`.
pub fn min_confs(&mut self, keychain: KeychainKind, n: u32) -> &mut Self {
    self.params.confirmation_policy.insert(keychain, n);
    self
}

[ValuedMammal]

Fixed by #258.