Feature request: Implement a `verity` flag and per-subvol root hashes

[Winterhuman]

It would be incredibly useful to have BTRFS support per-subvolume dm-verity style root hashes which can be used as a replacement for dm-verity, BTRFS already implements checksums for integrity (and fs-verity), however, there's no roothash option that can be specified like with dm-verity. Here's the proposal:

• Implement a verity flag that can be toggled per-subvolume. When enabled, btrfs subvolume show will display the subvol's root hash which is calculated periodically (or on write or some other trigger).
• Require verity subvol's to be mounted with mount -o subvol=veritysubvol,subvolhash=... similar to how roothash is needed for mounting verity partitions.

Related: https://github.com/kdave/btrfs-progs/issues/452#issue-1169868223