Support custom event handlers in `TransactionExecutor`

https://github.com/0xMiden/protocol/pull/3320 implements fee collection by requiring that a feature note and its sponsorship notes are direct neighbors. We should relax this assumption and instead add an event with a custom handler to find all sponsorship notes whose feature note ID matches the current note's ID. This requires defining a custom event and a corresponding custom event handler in `StandardsLib` and adding it to `TransactionBaseHost::core_lib_handlers` (and renaming the field). This should happen via the higher level types that wrap it.