Add license field

[accrazed]

Summary

This addition is with reference to #385, to add licensing information to the buildpack.toml file. I used this BP commit as reference. The main PR where this was discussed most is here.

Use Cases

As the issue states, this would allow users to see licensing information through their app image's BoM.

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).