Provide a mechanism for installing additional files provided by crates

Something like this is needed to install settings extensions. Each settings extension has a configuration file associated with it, e.g. [`motd.toml`](https://github.com/cbgbt/bottlerocket/blob/639de3dfdce12c34c428702ffa073c6c97c89a24/sources/models/settings-extensions/motd/motd.toml) as part of the [`motd` settings extension](https://github.com/cbgbt/bottlerocket/tree/639de3dfdce12c34c428702ffa073c6c97c89a24/sources/models/settings-extensions/motd).

When installing this settings extension in an RPM, we need for the binary to be placed under `%{buildroot}/usr/libexec/` and the associated config file to be placed under `%{buildroot}/usr/share/settings`.

We currently use something like:

```bash
cargo build --manifest-path 'sources/Cargo.toml' -p settings-extension-name
```

to build the extension binary, but we don't have any good way to discover the config file or tell cargo to install it into the buildroot.

----

It doesn't seem like cargo can do this, but one idea could be to provide a cargo extension command or a subcommand in twoliter that can read metadata in the `Cargo.toml` manifest which tells the tool how to install additional files into a given buildroot.

Start by reviewing the settings extension example at sources/models/settings-extensions/motd/motd.toml and the build command using sources/Cargo.toml. Compare the desired binary and configuration-file destinations under the buildroot with what Cargo and a possible twoliter subcommand can discover from Cargo.toml metadata. Done means a defined mechanism for installing the additional file alongside the extension binary.

Written by the indexing model from the issue text.