Support a configurable runtime

[lalten]

Right now the runtime is hardcoded to a prebuilt version (see #392) of the "official" "type-2" runtime.

This works fine, but there are actually quite a number of alternative AppImage runtimes that would be interesting to support.
I imagine this could be some config_setting / string flag to point rules_appimage at the label of the binary to be used as runtime executable blob.

Here are some examples of alternative runtimes off the top of my head:

• https://github.com/AppImage/type2-runtime (currently used via https://github.com/lalten/type2-runtime)
• https://github.com/xplshn/pelf
• https://github.com/VHSgunzo/uruntime
• https://github.com/mgord9518/shappimage
• https://github.com/AppImageCrafters/appimage-runtime
• https://github.com/AppImageCrafters/appimage-runtime-3

Possibly not direct replacements but still interesting:

• https://github.com/nimbus-build/kickoff (Bazel)
• https://github.com/blaizard/rules_bundle (Bazel)
• https://github.com/lalten/appimage-runtime-rs (Bazel)
• https://github.com/pkgforge-dev/ghostty-appimage
• https://github.com/solidsnack/arx (used in https://github.com/nix-community/nix-bundle)
• https://github.com/awesomebytes/EnvImage
• https://github.com/linux-packaging-rs/appimage-type2-runtime-rs

Having something like this would also open up the possibility of cross-platform support. (currently appimages are linux-only from depending fuse)