feat: win custom sign should have same signature of doSign

[link89]

• The win custom sign script should be able to use WinPackage as doSign does.
• Export doSign method to allow users to wrap it in their script instead of rewrite whole code signing logic. For example, to just skip some files to sign.

[link89]

@develar Can you help to review and approve this?
We found that after upgrade from v20 to v22, all binary files under app.asar.unpacked are signed, which would break some third party SDK, like this one #3637. So we hope that electron-builder can have some easy way to skip some files to sign.