Set per-user UID/GID to the work account in session containers

[achimnol]

There is a customer site that wants to use their internal SSO system and set Linux UID/GID from it as their storage system is also bound to those UID/GIDs.

To keep those custom storage management scheme, we need to revive the "unmanaged" vfolder which provides the mount of arbitrary host path in the agents like a vfolder.

Note

Unmanaged vfolders do not comply with Backend.AI's own quota management (quota scopes), and all such administrative settings should be manually done by the customer.
Also, they cannot be created or deleted via our UI. They just exist and only can be configured by administrators.

This issue is about to add a plugin hook to customize the container settings so that an SSO plugin could modify the UID/GID passed to the session entrypoint which creates the work user in session containers.

Related issues

• Prerequisite: Revisit the unmanaged vfolder with the latest storage-proxy architecture #2313
• Not mandatory but good to have: Allow running storage-proxy as root #2270