[Meta] Anomaly Detection: Universal Flow

[sean-zheng-amazon]

We are going to build unified workflow for realtime and historical anomaly detection within the same detector. So user doesn't need to create new historical detectors if they want to use the same realtime detector configuration to do historical anomaly detection, vice versa.

• [RFC] (RFC: Unified flow for realtime and historical anomaly detection opendistro-for-elasticsearch/anomaly-detection#380)
• [Universal flow feature change] (Single flow feature change #147)
• [Backward compatibility] ([Single Flow] backward compatibility  #133)

[kaituo]

we have already released the feature