feat: ios backup client state event

[Jasonvdb]

New RN event for monitoring backup client state.
Optional failure fields are included for temporary soft fails, BackupClient will continue to retry on it's own.

ldk.onEvent(EEventTypes.backup_state_update, (res: TBackupStateUpdate) => {
 //file: {
 //   lastQueued: number;
 //   lastPersisted?: number;
 //   lastFailed?: number;
 //   lastErrorMessage?: string;
 //}
}