Cargo Tree: New format string parameter

[timbess]

Add new format parameter to allow querying what platform each dependency is built for.

What does this PR try to resolve?

rules_rust is using cargo tree to query the dependency graph and create an equivalent internal graph of dependencies per architecture it's configured to build for. The issue I observed with their current implementation is that they were querying all build dependencies when querying for each target architecture, which caused host architecture only transitive build deps to be added to graph of the target architecture.

I tried querying with just build edges, but then we don't get to see the normal transitive deps under the build deps so we can't actually build a dependency graph of all the build deps accurately using filtering. Next I went to look through the format parameters and saw there wasn't really a way for me to extract what platform it's building for, hence this PR.

[rustbot]

r? @ehuss

rustbot has assigned @ehuss.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[epage]

While I appreciate this is a draft, we should likely reserve any conversations about "what is the problem", "what the the possible solutions", and "which solution do we want" to an issue.

[timbess]

@epage I can copy this to an Issue. I was just filling out the template Github presented when I opened the PR.

[epage]

For future reference, you can find out process at https://doc.crates.io/contrib/process/working-on-cargo.html#before-hacking-on-cargo