Add streaming_precision_recall_at_equal_thresholds

This helper method computes streaming tp, fp, tn, fp, precision, and recall for the user in a way that exhibits O(T + N) time and space complexity (instead of O(T * N)), where T is the number of thresholds and N is the size of the predictions tensor.

Thanks to Frank Chu for the efficient algorithm!

PiperOrigin-RevId: 172946073