[Pre-RFC] Proposal: bound Cargo's implicit upward discovery for config.toml files

I appreciate the comments!

Unfortunately, a config key would not solve the main use case for us. We build third-party repositories under a tool-managed environment, and we do not necessarily control or want to modify their .cargo/config.toml.

It also makes the boundary part of config discovery itself: Cargo would need to discover and load config files in order to know where config discovery should stop. That can maybe be designed as special side-band state, but for wrappers/tools we need the authoritative boundary to come from outside the discovered config graph.

I think a repository-owned marker like root = true could still be useful for projects that want to declare their own boundary. It just seems complementary to, rather than a replacement for, an invocation/tool-controlled boundary.