Support metadata compaction

[Fokko]

Feature Request / Improvement

Add support for compaction. This rewrites the existing manifests into a single one, reducing the number of calls to the object store. This should follow the Java configuration keys:

• commit.manifest-merge.enabled: Controls whether to automatically merge manifests on writes.
• commit.manifest.min-count-to-merge: Minimum number of manifests to accumulate before merging.
• commit.manifest.target-size-bytes: Target size when merging manifest files.

[HonahX]

I am interested in taking this if no one has started working on it.

[HonahX]

Based on offline discussion with @Fokko, I will first focus on implementing the MergeAppend which supports these keys

• commit.manifest-merge.enabled
• commit.manifest.min-count-to-merge
• commit.manifest.target-size-bytes

The MergeAppend will become the default append method since commit.manifest-merge.enabled is default to True.
The PR for MergeAppend is #363

BTW, it seems rewrite_manifest operations only depends on the commit.manifest.target-size-bytes. Shall we update the description to reflect this?