Update --ext=rust to support compiling the native extension from source

[karreiro]

What was the end-user or developer problem that led to this PR?

I needed to apply some tiny changes to the skeleton generated by the --ext=rust option to compile my native extension from source.

What is your fix for the problem, implemented in this PR?

When gem consumers install a gem with a native extension and their platform is not included the pre-compiled bundles, they need to compile it locally and rely on the extconf.rb file.

Referencing the extconf.rb file in the gemspec is required to make that work. We also need to include rb_sys as a dependency because the Rust extconf.rb depends on it.

Here's a demo of this change in action :)

Screen.Recording.2024-04-25.at.21.22.36.mov

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