Add APIs to return `WalletEvent`s when applying mempool transactions (or make `wallet_events` helper pub)

[tnull]

Describe the bug
In #336 we added the capabilities to return WalletEvents when applying blocks. However, we now stumbled across yet another omission: there currently is no API that returns WalletEvents when apply mempool transactions (i.e., for apply_unconfirmed_txs/apply_evicted_txs).

It would be great if these variants could be added, as we now had to resort to copy over the wallet_events helper method and generate the events on our end.

Alternatively, maybe the wallet_events helper could just be made a public utility method, and then all APIs returning events could be dropped again, as they just query tips and wallet transactions before and after and update and handing both to `wallet_events?

Expected behavior
Allow to retrieve WalletEvents for mempool transaction updates.

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____

Is this blocking production use?

• Yes
• No

Project or organization (optional)
LDK

[ValuedMammal]

there currently is no API that returns WalletEvents when applying mempool transactions

It strikes me as an anti-pattern to change every mutating method to return WalletEvents, first because we should be steering users toward using apply_update whenever possible, or at least change the internals to funnel all of the update logic through the apply_update API as a way to reason about the ways in which a wallet can be updated. Further I agree with adhering to more of an opt-in philosophy by enabling users to generate WalletEvents after batching any number of updates.

it would be great if these variants could be added

Possibly, although it does add to the overall API of Wallet.

Alternatively, maybe the wallet_events helper could just be made a public utility

Yes that sounds reasonable 👍

[evanlinjin]

As I suggested in #319 (comment) :

let old_view = wallet.view();
wallet.apply_update(update1)?;
wallet.apply_update(update2)?;
wallet.apply_block(&block, height)?;
let diff = old_view.diff(wallet.view());

This API will solve all problems without having to change all mutating methods. It will also allow wallet events to be opt-in and allow the caller to choose which snapshots to diff against.