vfs_ceph2: new VFS-module using libcephfs' low-level APIs

Ceph is a free and open-source software-defined storage platform that provides object, block and file storage on top a common distributed cluster foundation. CephFS is a POSIX-compliant file-system built on top of Ceph’s distributed object store, RADOS.

Libcephfs provides two types of programmable APIs: normal and low-level. The existing 'vfs_ceph.c' module uses the normal APIs while this new module uses the low-level one, which operates via inode and file-handle references, thus providing more fine grained granularity as well as potential for better performance.

The main advantages of using libcephfs low-level interfaces are:

• Explicit user-credentials (libcephfs' UserPerms) per call.
• Faster operation via (cached) inode or file-handle.
• Improved resource consumption on libcephfs internal Client.
• Asynchronous I/O (using ceph_ll_nonblocking_readv_writev, not impl yet).

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code