feat: add support for targets to cargo

[mhdawson]

Add support for targets in buildpack.toml which
are used for multi-arch support.

Summary

Adds support for targets in the buildpack.toml parsing.

Use Cases

This is needed
for multi-arch support. Otherwise Jam ends up stripping them out
when adding the version number to the buildpack.toml file before
creating the tgz and cnb for a buildpack

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).