Enable dependabot for automatic Rust dependency updates

[andreeaflorescu]

We are now moving to caret dependencies for rust-vmm crates. More context here (rust-vmm/vm-memory#199) about the reason.

We need to automatically update dependencies so that we don't have to do it manually when a new rust-vmm crate is released. The crates that generate lots of other updates are vmm-sys-util and vm-memory because they're used in lots of places.

The next step is also to add some sort of automation on releases.