Title: EGDS support

Description:
The current method of updating endpoints in a cluster through EDS is fine when the cluster size is relatively small, but when the cluster size is huge, suppose we have a 10000+ endpoints, and even a small amount of endpoints get changed it causes a full EDS push. Eventually this leads to a performance issue on Envoy and Pilot.

To fix this issue, Pilot (#19287) and the Envoy (#8400) community have initiated discussions and proposed to introduced into EGDS (Endpoint Group Discovery Service) protocol.

We are very happy to participate in the development of EGDS, a draft design document is available at below link. https://docs.google.com/document/d/1FFcK4eCndr0w2zt-5FLYd8hvVRL99zwJO13it6ng3lk/edit?usp=sharing

Thanks for your review and comments.

[optional Relevant Links:]
The design file for EGDS in Pilot is here.