fix: VFS Disk Cache Group Permissions, Merged Manifests Folder, is_mount checks

[baxeaz]

• Adding group owner permissions to vfs_disk_cache/cas_prefix.
• Writing merged manifests to new .vfs_manifests subfolder under session folder rather than /tmp.
• Using new VFSProcessManager.is_mount rather than os.path.ismount to propery check for mounts owned by job-user

What was the problem/requirement? (What/Why)

• The asset disk cache created under the session folder (session_dir/.vfs_object_cache) created a corresponding cas prefix when used (session_dir/.vfs_object_cache/my/cas/prefix) subfolder but did not apply group permissions to the entire folder structure before launching the vfs, preventing job-user access
• VFS Merged manifests were written out to a temporary file in /tmp and stuck around after session completion
• When merging manifests the os.path.ismount check was still used which does not properly return true for mounts owned by other users (such as when deadline-worker is checking for the mount owned by job-user)

What was the solution? (How)

• Add group owner permissions to the folder structure from the beginning of the asset disk cache to the leaf of the cas prefix
• Write merged manifests to a new session_dir/.vfs_manifests folder
• Switch os.path.ismount to VFSProcessManager.is_mount

What is the impact of this change?

• The on disk cache feature should work properly for deadline_vfs instances started by job-user
• Merged manifests should be properly cleaned up when the session folder is cleaned
• Manifests for existing mounts should merge properly

How was this change tested?

New test added which pass. Manual testing on CMF instances.

Was this change documented?

No

Is this a breaking change?

No