feat: combined callback

[0xl3on]

Motivation

Closes #326

Solution

Hey, here's a proposed solution to the issue:

Added a CombinedCallback<A, B> that pairs two callbacks and merges their handles, plus a with_callback builder on LogCallback:

  let callback = LogCallback::new(rpc)
      .send_fcu(true)
      .with_callback(MyCustomCallback);

Both sides run on every on_tx_sent / on_batch_sent, so custom callbacks get the DB caching for free. No trait changes, NilCallback and --ignore-receipts still behave the same

PR Checklist

• Added Tests
• Added Documentation
• Ran cargo +nightly clippy --workspace --lib --examples --tests --benches --all-features --locked --fix
• Ran cargo fmt --all
• Note breaking changes in PR description, if applicable
• update changelogs
  • Update CHANGELOG.md in each affected crate
  • add a high-level description in the root changelog

[zeroXbrock]

thanks @0xl3on, this is great!
I just added a lil trait (IntoCombinedCallback) + generic impl to add a with_callback method to all impls of SpamCallback so any callback could easily chain more callbacks, making use of CombinedCallback in the background. Now lib users can just do something like mycallback.with_callback(another_callback), and as long as they implement SpamCallback it'll work 😎