Enhancement: Make overwrite behavior of `restore` customizable

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

- `--overwrite always` (default): Always overwrites already existing files.
  The `restore` command will verify the existing file content and only restore
  mismatching parts to minimize downloads. Updates the metadata of all files.
- `--overwrite if-changed`: Like `always`, 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
  like with `always`. Updates the metadata of all files.
- `--overwrite if-newer`: Like `always`, but only overwrites existing files
  when the file in the snapshot has a newer modification time (mtime) than the
  existing file.
- `--overwrite never`: Never overwrites 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
https://github.com/restic/restic/pull/4864
https://github.com/restic/restic/pull/4921
