Add feature to enforce licence metadata

[reidbaker]

Add a feature to enforce that skills have a licence file and that the licence metadata matches a list of approved licences.
Will include

• A new command line flag to enforce the presence of the licence metadata entry in frontmatter.
• A new command line flag to pass a list of approved licence files that the metadata entries must match.
• A new command line flag to print deduplicated list of licence values for all the skills analyzed
• A new yaml config entry for enforcing licence metadata presence
• A new yaml config entry for enforcing that the licence entry matches one of a list of approved licence files

[reidbaker-agent]

Moved to google/skills_lint.dart#78.