Add support for selfhosted Keygen

[hrueger]

• Electron-Builder Version: v25.1.8

• Node Version: v22.3.0

• Electron Version: 30.0.9
• Electron Type (current, beta, nightly): current

• Electron Updater Version: 6.3.9

• Target: MacOS

Hi,
we're using a selfhosted Keygen, however, the server hostnames are hardcoded here and here.

I'd like to be able to set this in the package.json or the electron-builder.json, that would be great!