Enhancement: Cache blobs read by the `dump` command

When dumping a file using the `dump` command, restic did not cache blobs in any
way, so even consecutive runs of the same blob were loaded from the repository
again and again, slowing down the dump.

Now, the caching mechanism already used by the `fuse` command is also used by
the `dump` command. This makes dumping much faster, especially for sparse files.

https://github.com/restic/restic/pull/3508
