unified format API of stateful session header/cookie value

*Title*: *format API of stateful session header/cookie value*

*Description*:
Envoy added new proto format session cookie in the https://github.com/envoyproxy/envoy/pull/26761. And gPRC also add a new format for the weighted clusters session support at https://github.com/grpc/proposal/blob/master/A60-xds-stateful-session-affinity-weighted-clusters.md

At the initial design of stateful session filter, I just want provide a simple way to override the upstream load balancing and I didn't thought it would be used in more complex scenarios. So, only simple address string is used in the session cookie in the initial design.

Considering latest changes to the stateful session filter (in both Envoy and gRPC), I think we should give a formal discussion about following questions:

1. should a unified format be used for session header/cookie?
2. should the format be part of our standard xDS API?

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