[DRAFT] [ADD] Swift Package Manager Support

[mflknr]

Hello,

this PR tries to add SPM support in addition to CocoaPods since these are in maintenance mode. The source code compiles and works (as far as i can tell), but I hit a few roadblocks where I figured to ask for some guidance.

1. The public headers: The podspecs defines the "Public" folder as the path where the public headers are located. Yet, in libraries were Criteo is used with pods, i observed that ALL headers are available and therefor public. The package file seems to be more strict and only makes the header in the "Public" folder public. Thats why I initially said in Swift Package Manager Support #94 that I cannot use the xcframework binary because the headers are not complete. They are complete, but just differently available over pods. It is working now, but I need feedback if everything is semantically correct.
2. Tests: I had to move some files and therefor targets back and forth because SPM does not support swift/objc mixed targets (which was really bad tbh). The original testplan are no longer working and I cant figure out what was tested there. So someone has recreate the testplans from scratch. We also run into the same problem with the public headers here ...
3. Automation: Without working testplans, I cant even try to setup fastlane again. So ... there is that.

Anyway, I really need feedback here to continue on this. Thanks.

Closes #94