:sparkles: Marker support for Rust file extension .rs

[mabulgu]

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I have a new Operator SDK plugin for Rust*, and I use kubebuilder markers for scaffolding. This PR adds the .rs extension support to the marker.

Here is the plugin repo: https://github.com/SystemCraftsman/rust-operator-plugins

I would like to suggest this to be included in the Operator SDK org later on (gonna do that via the issue that I will open there).

Also I see the message When adding additional file extensions, update also the NewMarkerFor documentation and error, but no idea where I can update these. If anyone can show me the way, I can also update them to include Rust file ext.

(*) I recently presented the plugin in a KCD event: https://speakerdeck.com/mabulgu/building-kubernetes-operators-in-rust-a-new-operator-sdk-plugin