Explore `Wallet` modularization

[tnull]

We increasingly get user requests interested to use LDK Node, but based on a custom wallet implementation rather than on the baked-in BDK.

We should therefore explore whether it's feasible to modularize Wallet and offer the BDK wallet as a default implementation.

While a Wallet trait may be tempting, it's a bit unclear how/if this could be exposed in bindings. Possibly UniFFI callback interfaces could be used, but it might put several restrictions on our design, e.g., unclear whether we can inherit from upstream traits such as WalletSource, etc.

[tnull]

Closing as 'not planned' for now, can always revisit if there is particular user demand.