Cargo design doc

[brunoapimentel]

Early draft of the design doc for implementing Cargo support. There are a few sections still marked as TODO, most notably all the commentary on the Cargo+Pip support. So don't waste too much time in the details, this is bound to change quite a bit still.

It is also worth checking the test repo: https://github.com/cachito-testing/cachi2-rust. Same as this design doc, it is still a WIP.

I opened this as a PR because the length of the document would make it a little bit more unconfortable to review in the Github discussions format, and also to test if PRs could be an overall better way to handle design docs and other discussions.

Maintainers will complete the following section

• Commit messages are descriptive enough
• Code coverage from testing does not decrease and new code is covered
• Docs updated (if applicable)
• Docs links in the code are still valid (if docs were updated)

[eskultety]

it'll be a while before I get to read/understand/review this, but kudos for pushing the "PR for designs" way. Feels like it's going to be more ergonomic + we can not only close the PR, but also retain the doc for future reference, OR convert it to docs, i.e. it gives us a bit more flexibility.