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

# 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`.