Implement `rcodesign` support for code signing on Linux

Recently Streamplace has been able to move almost all of our Mac building over to Linux containers [thanks to the `apple-codesign` Rust package, AKA the `rcodesign` command-line tool](https://gregoryszorc.com/docs/apple-codesign/0.17.0/apple_codesign_rcodesign.html). This has saved us money on Mac builders and made our builds faster.

Unfortunately we still have a dependency on a Mac builder for our Electron app due to this package only supporting native Mac codesigning. It would be amazing if `electron-osx-sign` implemented `rcodesign` support! Would be very helpful.

(We're stuck with macOS in CI for the foreseeable future for our iOS builds... [but maybe that can change too someday](https://github.com/xtool-org/xtool)!)

Start by reading the electron-osx-sign implementation and the linked rcodesign documentation, then trace where native macOS signing is selected. Determine the integration points and validation needed for Linux containers. Done means an Electron app can be signed through rcodesign without requiring a Mac builder.

Written by the indexing model from the issue text.