Async commit and merge: lakectl

Update lakectl to call the async API and poll for status, with a fallback to the existing sync operations for the OSS.

Follow the [design doc](https://github.com/treeverse/dev/blob/1884affbe35b0de47036127efe0c0e17a97d4969/designs/open/async-commits-merge/async-commits-merge.md).