Make dependency license an interface

[sophiewigmore]

Summary

Will resolve #222.
With version v0.14.1 of packit and beyond, any buildpacks that do not have a buildpack.toml that matches what we expect in packit, jam summarize fails because the license field (slice of strings) is represented as as struct.

This has been an issue in our language family buildpacks (https://github.com/paketo-community/rust/actions/runs/1225798554).

This change just makes the dependency license field in cargo.Config into a slice of interface rather than a slice of string. This makes the field more flexible so that commands in jam that consume cargo.Config (such as jam summarize) will not error out with cannot unmarshal object into Go struct field Config.metadata of type string.

This change should have no effect on buildpacks that have the buildpack.toml format we expect.
This will be followed by a PR in jam to slightly modify how dependency licenses are gathered.

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).