Define V2 stone format

[ikeycode]

Tracking issue to settle on the absolute definition and requirements for the V2 stone format.

Basic requirements:

• Restoration of timestamps
• xattr support for SELinux labels etc.
• Addition of digests into stones for fsverity
• New type for the versioned index (versioned repos)
• Signature payload (chained multisig)
• Improved access patterns

[ikeycode]

Note the intention is to break this issue between V3 and V2 and determine the absolute basic requirements to attain the goals of immutable OS and improved experiences. Across V2 and V3 we will transition to being immutable + fsverity (without compromising on our atomic vision)