Preserve Backward compatibility in 0.8.0 for #1144

[sungwy]

In #1144, we moved 'public' classes and functions to submodules.

Although 'public' by naming convention, these classes and functions are meant for internal use. The current lack of explicit external API declarations makes it difficult to communicate with our end-users, which classes and functions we are intending to support for external use. (https://github.com/apache/iceberg-python/issues/1099)

The classes that were moved and are not referenced as an import in pyiceberg.table module are as follows:

• Move
• MoveOperation
• DeleteFiles
• FastAppendFiles
• MergeAppendFiles
• OverwriteFiles
• UpdateTableMetadata

UpdateTableMetadata is an abstract base class, and hence cannot be used directly.

This PR proposed to rename these as 'private' classes with single underscore prefix, and introduces functions that return these classes with a deprecation warning. This will allow users who are for some reason using these classes to see the warnings and update their code to not use these classes anymore.

Thanks @kevinjqliu for flagging this as a potential issue for backward compatibility.