upstream: add EDS indirection to allow repeated hosts (#5432)

Adds the API for an additional EDS indirection that allows endpoints to
specified outside the LB structure. This opens up for being able to
reference the same endpoint multiple times in a single CLA.

Risk Level: Low, only API changes for now
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

#4280

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ b49e37989160913e91a42824c91b664de82e7a05

envoy/api/v2/eds.proto

@@ -53,6 +53,9 @@ message ClusterLoadAssignment {
   // List of endpoints to load balance to.
   repeated endpoint.LocalityLbEndpoints endpoints = 2 [(gogoproto.nullable) = false];
 
+  // Map of named endpoints that can be referenced in LocalityLbEndpoints.
+  map<string, endpoint.Endpoint> named_endpoints = 5;
+
   // Load balancing policy settings.
   message Policy {
     reserved 1;

envoy/api/v2/endpoint/endpoint.proto

@@ -54,8 +54,11 @@ message Endpoint {
 
 // An Endpoint that Envoy can route traffic to.
 message LbEndpoint {
-  // Upstream host identifier
-  Endpoint endpoint = 1;
+  // Upstream host identifier or a named reference.
+  oneof host_identifier {
+    Endpoint endpoint = 1;
+    string endpoint_name = 5;
+  }
 
   // Optional health status when known and supplied by EDS server.
   core.HealthStatus health_status = 2;