Draft: Proof of concepts: Implement Policy#abilities_missing callback

Description

This is a proof of concepts for #25 which implements an abilities_missing hook.

See also gitlab-org/gitlab#396352.

Draft?

• Integrate the approach with Support::AbilityChecker in gitlab-org/gitlab
• Add specs
• Add docs
• Check performance impact

Related Issues

See: #25

Suggested version bump

• Major (backwards incompatible changes)
• Minor (backwards compatible changes)
• Patch (API compatible changes)

Checklist

• Tests have been added or updated to cover any changes in behavior
• This does not change the API to consume this library, or a suggested version bump has been provided
• The CHANGELOG.md has been updated
• No new runtime dependencies have been introduced