[API] Allow routing to the cluster specified in the request_header for weighted cluster case. (#17673)

- Add cluster header to weighted cluster proto. This enables the ability of routing to the cluster specified in the request_header for weighted cluster (i.e. multiple upstreams) case. name and cluster_header will be oneof cluster_specifiter field in the next release. Currently, they can only be regular fields due to compatibility issue.
- Add the validation logic (PGV annotation + runtime check)
- Add 3 unit tests for invalid weighted cluster configs.
Note: I have implemented the core implementation and tested it with end-to-end
integration test/example locally. Those changes will be in a separate PR once this API change is approved.

Risk Level: Low
Testing: CI, added an integration test for implementation of this API locally

Signed-off-by: Tianyu Xia <tyxia@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ef70185f3ac241d09eabfe15c65b0d631134f74f

envoy/config/route/v3/route_components.proto

@@ -312,7 +312,7 @@ message Route {
 message WeightedCluster {
   option (udpa.annotations.versioning).previous_message_type = "envoy.api.v2.route.WeightedCluster";
 
-  // [#next-free-field: 12]
+  // [#next-free-field: 13]
   message ClusterWeight {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.api.v2.route.WeightedCluster.ClusterWeight";
@@ -321,9 +321,31 @@ message WeightedCluster {
 
     reserved "per_filter_config";
 
+    // Only one of *name* and *cluster_header* may be specified.
+    // [#next-major-version: Need to add back the validation rule: (validate.rules).string = {min_len: 1}]
     // Name of the upstream cluster. The cluster must exist in the
     // :ref:`cluster manager configuration <config_cluster_manager>`.
-    string name = 1 [(validate.rules).string = {min_len: 1}];
+    string name = 1 [(udpa.annotations.field_migrate).oneof_promotion = "cluster_specifier"];
+
+    // Only one of *name* and *cluster_header* may be specified.
+    // [#next-major-version: Need to add back the validation rule: (validate.rules).string = {min_len: 1 }]
+    // Envoy will determine the cluster to route to by reading the value of the
+    // HTTP header named by cluster_header from the request headers. If the
+    // header is not found or the referenced cluster does not exist, Envoy will
+    // return a 404 response.
+    //
+    // .. attention::
+    //
+    //   Internally, Envoy always uses the HTTP/2 *:authority* header to represent the HTTP/1
+    //   *Host* header. Thus, if attempting to match on *Host*, match on *:authority* instead.
+    //
+    // .. note::
+    //
+    //   If the header appears multiple times only the first value is used.
+    string cluster_header = 12 [
+      (validate.rules).string = {well_known_regex: HTTP_HEADER_NAME strict: false},
+      (udpa.annotations.field_migrate).oneof_promotion = "cluster_specifier"
+    ];
 
     // An integer between 0 and :ref:`total_weight
     // <envoy_v3_api_field_config.route.v3.WeightedCluster.total_weight>`. When a request matches the route,