feat: Add support for targets in cargo for extension

[pacostas]

Summary

While jam packages an extension, it recreates the extension.toml. The generated extension.toml is based on what has been parsed from packit. In case some of the variables available on the extension.toml are not available for parsing from the config, will not be included on the generated extension.toml.

This PR adds the targets attribute on the config, similar to this PR for buildpacks #604, to packit be able to output them on the generated extension.toml.

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).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: pacostas / name: Costas Papastathis (23f790c)

[pacostas]

I changed pc and the default git configs were not correct, which is why the CLA was not passing. PR is ready for review @paketo-buildpacks/tooling-maintainers