Add reactor-rust as a submodule

[lhstrh]

We plan to publish release notes for our runtime implementations as part of the Lingua Franca release notes. This will give the runtime implementations more visibility, and it compensates for a lack of versioning/releases of happening in their repositories. Currently, all our runtime implementations are hooked into the Lingua Franca repository as a submodule, _except for reactor-rust, for which we keep track of its refs in a separate file. To make it easier to automatically generate and include release notes for reactor-rust, we should also include it as a submodule.

Note the command git submodule can be used to find the ref a submodule points to.

Going forward, we should always include a runtime used by LF as a submodule, unless the runtime maintains its own versioning/releases.

If we decide that using a submodule for reactor-rust is not a good fit, we can also resolve this by starting to do proper versioning for it and publish release notes separately.

[cmnrd]

Just to add a few thoughts: Currently we let cargo resolve reactor-rust. Maybe it would be possible to let cargo default to the version of reactor-rust that is shipped with lfc (as a submodule) and let it only resolve reactor-rust from its git repo if the user explicitly specifies a different runtime version. This would be analogous to the current setup for C++, where we use the builtin version of reactor-cpp by default, but we can also specify another version or use one that is installed to the local system.