Allow specifying extra command-line flags for prune

[nsubtil]

Is your feature request related to a problem? Please describe. When backing up using glacier S3 storage (by passing -o s3.storage-class=GLACIER as a repo flag), restic will manage the storage class for each pack file such that index data remains accessible, but file data is kept in glacier. This allows most operations to work, but makes the repo largely append-only.

The --repack-cacheable-only prune flag works around the limitations of glacier by not attempting to repack any data in glacier storage. Entire packs can still be deleted, which is often enough to prevent the repo from growing unbounded in practice. This is much more desirable than restoring everything from glacier to do the prune, since it avoids significant costs on most S3 providers.

Unfortunately we can't just add this to the general flags for the repo, as other commands will fail if this flag is set, and backrest does not have another way to specify this flag.

Describe the solution you'd like A simple solution could be to support specifying additional repo-level command line options that are specific to prune.

(A more elaborate future direction could be to support template configs, and have a template set up to work reasonably well with glacier storage by default.)