This PR introduces the installation of the `musl`-based version of Rust,
in order to support alpine-based runtime environments (Rust is used by
[RedisJSON](https://github.com/RedisJSON/RedisJSON)).
A new BUILD_WITH_MODULES flag was added to the Makefile to control
building the module directory.
The new module directory includes a general Makefile that iterates
over each module, fetch a specific version, and build it.
Co-authored-by: YaacovHazan <yaacov.hazan@redislabs.com>
