Simplify chainhook-sdk interface

[lgalabru]

Simplifying the chainhook-sdk would be helpful for helping developers building new indexers on top of the chainhook-sdk. Something like:

let indexer = Indexer::new();

indexer.apply_block(|event| {
    // do something
});

indexer.rollback_block(|event| {
    // do something
});

indexer.run();

[lgalabru]

Foundation implemented in https://github.com/hirosystems/chainhook/pull/362