Incrementally backup btrbk-cretead snapshots into object storage services.

Snapshot subvolumes are named NAME.TIMESTAMP[_ID]

1. parent backups: btrfs subvolume copied as is, with btrfs send SUBVOLUME
   • named NAME.TIMESTAMP[_ID].
2. backup backup: btrfs subvolume sent incrementally, with btrfs send -p BASE_SUBVOLUME SUBVOLUME
   • named NAME.TIMESTAMP_ID-BASE_TIMESTAMP_[ID]

(in OSS there can be file splitting / encryption / compression. )

0. user should specify NAME.TIMESTAMP 1a. if not found, fail 1b. if the backup is a full one, do a oss-read | btrfs receive 1c. if the backup is a incremental one, restore its parent

[volumes]

[storage.x]
vendor=X # opendal-supported 

# list stat
btrbkar status

btrbkar backup NAME_OR_PATH

btrbkar restore NAME DEST

-c CONFIG -b BACKEND --no-prune only rir bbkar SNAPSHOT_PREFIX , _btrbk_snapshots/NAME

The idea is inspired by this comment, btrfs-send-to-s3 and others.

• encryption (maybe with https://github.com/str4d/rage/) and compression