[Draft] Decomposed based posix filesystem driver

[aduffeck]

This PR adds a new posix fs driver which stores files and metadata directly on a posix compliant directory. It also adds some special handling for gpfs, i.e. for improving the notifications about changes to the underlying data.

It is based on decomposedfs but uses a copy of the lib with some changes that require some refactoring before it can be merged back to the "real" decomposedfs code. We should consider doing this before merging the pr in order to avoid code cuplication.

Supported Features:

• Personal and project spaces
• Basic file operations, sharings, previews, etc.
• "Assimilation" of new data being added to the filesystem bypassing ocis (using either inotifywait, gpfs file audit logs or gpfs watchfolder queues)
• Handling of changes being made to the files on disk (renaming, moving, etc.)

Currently missing:

• Trashbin support
• File versioning
• uid mapping