Enhancement: Cache blobs read by the dump command

When dumping a file using the `restic dump` command, restic did not cache blobs
in any way, so even consecutive runs of the same blob did get 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