To assist with the future PBS->QBS app migration feature, add the realm core version to the BSON data in the body of the app login request (where the platform and SDK version info is provided) so the server can provide information to the user regarding how many clients are still using an older (unsupported) version of the client that does not support PBS->QBS migration. It can also be added to the (future) metrics that are reported by the server regarding which core versions are currently in use.