Add option for macOS .dmg FTP releases

[BPerlakiH]

Based on the Kiwix CD, we could start supporting custom apps macOS FTP releases in the .dmg format.

[kelson42]

@BPerlakiH How much work would that be?

[BPerlakiH]

@kelson42 Theoretically it should be a rather quick one.
We have the README.md already containing how to obtain the missing credentials (eg. for DWDS),
see: https://github.com/kiwix/kiwix-apple-custom/blob/main/README.md#create-a-development-signing-identity-optional (the optional parts).

So we need to set that up for DWDS, and possibly with minor modifications borrow the FTP steps from the Kixix CD workflow.

From this point of view it's relatively small scope.

More investigation is needed to finally figure out, why those macOS FTP builds are still "flaky", and not always passing the first time.

So we can do a fix for that and bring it into this repo, or do it the other way around and bring what we have, it's just that I will need to press re-try from time to time on both CD processes.