Ignore unknown fields in manifest with `--force` (#495)

Ignore unknown fields in manifest with `--force` (#495)

* Use manifest source (local, index) for finer errors

* Generate `metadata-version` during release saving

* Script to prepend manifest metadata versions

Not really necessary since we don't expect index manifests to include such
metadata version. Still, it makes clear the change.

* Testsuite adjustments for extra error output

Since we now take advantage of the manifest provenance (index/local), some
error messages require tweaking.

* Review: remove `metadata-version` field

* Allow unknown metadata fields with --force

Usually new fields will correspond with new features. Ignoring these from an
old `alr` version might be good enough for some commands to still work.

* Test for ignoring unknown properties