Add support for rust via cargo

[jonabc]

This change adds support for rust using the cargo package manager. Generally this is one of the simplest source implementations in a long time 🎉 . A HUGE HUGE thank you to the rust and cargo developers for providing all the metadata this tool needs from a single cargo metadata CLI call, and returning data in an easy to consume format.

There is an extensibility option available with cargo.metadata_options configuration. This key can be set to a string or an array of strings, and will be appended to the end of the default CLI call cargo metadata --format-version=1.

The source is pretty straightforward - it pulls the ids from resolved.nodes output and matches them up to package data given in packages, excluding any packages who's ids are present in workspace_members as 1st party packages that don't need reporting on. All of the metadata that licensed needs is available in the packages object ❤️ .