Track Cargo.lock in git

[jpbrucker]

Enable creating a buildroot package for keybroker-app.

To build packages deterministically, buildroot needs the Cargo.lock file to precisely describe all dependencies. In general the cargo doc recommends checking Cargo.lock into the repo:

https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html

Cargo.lock is automatically generated on build, and changes alongside Cargo.toml.

[jpbrucker]

Note that this will soft conflict with other PRs that update Cargo.toml (#16). If they are merged first I'll rebase this patch.

[Arnaud-de-Grandmaison-ARM]

While I totally agree with @jpbrucker, I have #17 that will be pushed as soon as #16 is merged. It affects all the Cargo.toml in the repository by moving the dependency management to the top Cargo.toml and making the individual projects inherit from the central dependencies and use the piece they need. I would thus suggest to merge this PR once #16 and #17 have been merged. Our Cargo.toml / Cargo.lock story will then be in a good shape !