manifest: Add rust lang module

Add zephyr-lang-rust as an optional module.  This module brings in
support for writing applications in Zephyr using the Rust programming
language.

At this point, the integration supports:

- Building applications with Rust support.
- CI testing of samples and tests from the rust module (twister
  functionality already present).

Still to do is adding documentation to the Zephyr docs with an overview
of Rust support as well as the generation of the rustdoc docs.

Signed-off-by: David Brown <david.brown@linaro.org>