Add `--known-missing` and `--known-missing-file` options

Once in a while, objects are missing from the storage. It should not happen but it does: previous removals, replayer errors, corruptions…

swh alter remove and swh alter recovery-bundle resume-removal now accepts the new options --known-missing and --known-missing-file to handle missing objects.

--known-missing is followed by a SWHID and can appear multiple times. --known-missing-file is followed by the path (or - for stdin) of a file with one SWHID per line (skipping empty lines or lines starting with #).

Known missing objects will be skipped during the “inventory” phase and mark as unremovable during the “removable” phase. Due to the latter, they will not be used when creating the recovery bundle, and skipped from removal as well.

Depends !41 (merged)