[feat] Config option for DMG Volume icon

[Blatman]

Describe the problem

Following on from #4669 which has recently been closed/merged it would be good to have a config option for the Volume icon rather than using the first .icns file in the folder which is the general app icon - the advantage being that the DMG will be readily identifiable as a DMG rather than an app.

Describe the solution you'd like

Work effort should be similar to #4669

Alternatives considered

Separate DMG bundling can be done with more hassle but this change would be a good value add to the existing build/bundle process which largely runs well

Additional context

No response

[Blatman]

Just thinking that maybe all this would require is a change in the bundle_dmg script to use say dmg.icns rather than icon.icns.
This would mean that the dmg.icns could be adapted to suit the devs preference.