Support merge manifests on writes (MergeAppend)

[HonahX]

Add MergeAppendFiles. This PR will enable the following configurations:

• 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.

Since commit.manifest-merge.enabled is default to True, we need to make MergeAppend as the default way to append data to align with the property definition and java implementation