Warn the user when unrecognized image meta is dropped.

**Describe the problem/challenge you have**
When image meta is being parsed from an upstream `imgpkg` lock file,
if the structure of the meta is unknown to `kbld` (e.g. the meta was created by a newer version of `kbld` than the one being used to deploy),
then that meta is discarded without warning.

**Describe the solution you'd like**
While parsing meta, if one is unrecognized:
- with normal output, give one general warning (e.g. "There was some resolution history that was unrecognized and therefore discarded. This had no effect on successfully resolving image references. Run again with --debug for details.")
- if the user supplies the --debug flag (which doesn't exist at the time of writing), then report each unrecognized meta.

**Anything else you would like to add:**

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Start by locating the image-meta parsing entry point for upstream imgpkg lock files and the existing command output and flag handling. Trace how unrecognized metadata is discarded, then verify that normal output gives one warning, while debug output reports each discarded item; done means image resolution remains unchanged and both warning modes are covered.

Written by the indexing model from the issue text.