upstream: implement Cluster's load_assignment field (#3864)

This patch implements load_assigment field in CDS' Cluster.
This change specifically adds the implementation of the new load_assigment field
for clusters with discovery-type: STATIC, STRICT_DNS and LOGICAL_DNS.

While adding this load_assigment field implementation to Cluster,
this patch also allows specifying optional (active) health check config per specified upstream host.

Risk Level: medium
Testing: unit tests
Docs Changes:

This unhides docs for endpoint health check config
Release Notes: N/A

Fixes #439

Signed-off-by: Dhi Aurrahman <dio@rockybars.com>

Mirrored from https://github.com/envoyproxy/envoy @ b32eabfc141760ec14622a4a2a2f0ab0a741cd6c

envoy/api/v2/cds.proto

@@ -160,7 +160,13 @@ message Cluster {
   // :ref:`STRICT_DNS<envoy_api_enum_value_Cluster.DiscoveryType.STRICT_DNS>`
   // or :ref:`LOGICAL_DNS<envoy_api_enum_value_Cluster.DiscoveryType.LOGICAL_DNS>`,
   // then hosts is required.
-  repeated core.Address hosts = 7;
+  //
+  // .. attention::
+  //
+  //   **This field is deprecated**. Set the
+  //   :ref:`load_assignment<envoy_api_field_Cluster.load_assignment>` field instead.
+  //
+  repeated core.Address hosts = 7 [deprecated = true];
 
   // Setting this is required for specifying members of
   // :ref:`STATIC<envoy_api_enum_value_Cluster.DiscoveryType.STATIC>`,
@@ -176,7 +182,6 @@ message Cluster {
   //   :ref:`endpoint assignments<envoy_api_msg_ClusterLoadAssignment>`.
   //   Setting this overrides :ref:`hosts<envoy_api_field_Cluster.hosts>` values.
   //
-  // [#not-implemented-hide:]
   ClusterLoadAssignment load_assignment = 33;
 
   // Optional :ref:`active health checking <arch_overview_health_checking>`

envoy/api/v2/endpoint/endpoint.proto

@@ -29,7 +29,7 @@ message Endpoint {
   //   and will be resolved via DNS.
   core.Address address = 1;
 
-  // [#not-implemented-hide:] The optional health check configuration.
+  // The optional health check configuration.
   message HealthCheckConfig {
     // Optional alternative health check port value.
     //
@@ -40,8 +40,8 @@ message Endpoint {
     uint32 port_value = 1;
   }
 
-  // [#not-implemented-hide:] The optional health check configuration is used as
-  // configuration for the health checker to contact the health checked host.
+  // The optional health check configuration is used as configuration for the
+  // health checker to contact the health checked host.
   //
   // .. attention::
   //