convert: add support for --runtime-features like mkfs

[kdave]

For parity, add -R/--runtime-features, which is just space_cache=v2 (free-space-tree) for now. The basic infrastructure is missing so it needs the command option and passing around the flags (easy).

The catch: the filesystem creation phase is different in convert than in mkfs so it's not sufficient to set mkfs_cfg.runtime_features.