Generate SharedId and send in auction requests

[jclou]

I based the implementation on this proposal #1057 with a few changes:

1. The consent checks and id generation/deletion is done lazily when the .sharedId property is accessed. This ensures that the latest consents are always checked before usage. However we still want to give app developers control over whether sharedId is used, so I added the flag sharedIdEnabled that developers need to set before we generate/send SharedIds. Maybe we want to also allow app developers to clear a previously generated id, but I did not implement this.
2. Since ATT deprecates and replaces LMT, we only need to check LMT as a fallback for older versions of iOS where ATT is not yet available (e.g. iOS 13 and below).