Add typedMetadata for RouteEntry&ClusterInfo (#4723)

Add typedMetadata interface for RouteEntry and ClusterInfo.

Design doc: https://docs.google.com/document/d/1AKA_5iMSR6WZaDbjQfLFbK9maO_la_MZgVtYdD9gWtA/edit?usp=sharing

Risk Level: LOW (new interface)
Testing: unit test.
Docs Changes: Api level.

Fixes #4513

Signed-off-by: Xin Zhuang <stevenzzz@stevenzzz6.cam.corp.google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 13789ee7ebbd2f0827ec71ed6c885bd4c67c08ca

envoy/api/v2/core/base.proto

@@ -75,8 +75,8 @@ message Node {
 }
 
 // Metadata provides additional inputs to filters based on matched listeners,
-// filter chains, routes and endpoints. It is structured as a map from filter
-// name (in reverse DNS format) to metadata specific to the filter. Metadata
+// filter chains, routes and endpoints. It is structured as a map, usually from
+// filter name (in reverse DNS format) to metadata specific to the filter. Metadata
 // key-values for a filter are merged as connection and request handling occurs,
 // with later values for the same key overriding earlier values.
 //
@@ -84,6 +84,9 @@ message Node {
 // http_connection_manager in the envoy.http_connection_manager.access_log
 // namespace.
 //
+// Another example use of metadata is to per service config info in cluster metadata, which may get
+// consumed by multiple filters.
+//
 // For load balancing, Metadata provides a means to subset cluster endpoints.
 // Endpoints have a Metadata object associated and routes contain a Metadata
 // object to match against. There are some well defined metadata used today for