Enhancement: Make overwrite behavior of `restore` customizable

The `restore` command now supports an `--overwrite` option to configure whether
already existing files are overwritten. This behavior can now be configured via
the `--overwrite` option. The following values are supported:

* `--overwrite always` (default): always overwrites already existing files. `restore`
  will verify the existing file content and only restore mismatching parts to minimize
  downloads. Updates the metadata of all files.
* `--overwrite if-changed`: like the previous case, but speeds up the file content check
  by assuming that files with matching size and modification time (mtime) are already up to date.
  In case of a mismatch, the full file content is verified. Updates the metadata of all files.
* `--overwrite if-newer`: only overwrite existing files if the file in the snapshot has a
  newer modification time (mtime).
* `--overwrite never`: never overwrite existing files.

https://github.com/restic/restic/issues/4817
https://github.com/restic/restic/issues/200
https://github.com/restic/restic/issues/407
https://github.com/restic/restic/issues/2662
https://github.com/restic/restic/pull/4837
https://github.com/restic/restic/pull/4838