Create extension trait that extends `Wallet` to return `InputCandidates`

[evanlinjin]

Rationale

This is a temporary measure so that users of Wallet can more easily start using bdk_tx before the API stabilizes. Additionally, this will make it easier to build FFI bindings with this available.

Requirements

• Introduce a feature flag wallet_ext (or something similar).
• The extension trait should have two methods: One for RBF, one for general coin selection.
• RBF method should take in a set of transactions to replace. If there are descendants of this set (that aren't included in the set), we should return an error. The caller must explicitly opt-in to replacing all transactions.

Helpful Tips

Inspiration can be drawn from examples/common.rs.

[aagbotemi]

@evanlinjin I would love to work on this.