Add VersionTrackerStep to Common Steps in the SDK

[dermanyang]

Purpose

Introduce the LatestVersionProcessedTracker as a common step in the SDK so processors across in aptos-indexer-processors and aptos-indexer-processor-example can reuse the same code.

Example usage in PR#8 of the example repo:

let version_tracker = VersionTrackerStep::new(
            self.config.processor_config.name().to_string(),
            ProcessorStatusSaverImpl {
                conn_pool: self.db_pool.clone(),
            },
            UPDATE_PROCESSOR_STATUS_SECS,
        );
...

Technical Overview

To prevent a migrations/ directory in the SDK which would cause many forks an adapter trait object, ProcessorStatusSaver with method save_processor_status, is used.

Testing

Tested manually in examples repo by refactoring the events processor to use this step: aptos-labs/aptos-indexer-processor-example#8