xds: build the interceptor chain for routes with cluster_specifier_plugins as well

[easwars]

We are currently building the interceptor chain only for routes that point to weighted_clusters. See:

grpc-go/internal/xds/resolver/xds_resolver.go

Line 374 in c7ec4d9

interceptor, err := newInterceptor(r.xdsConfig.Listener.HTTPFilters, wc.HTTPFilterConfigOverride, rt.HTTPFilterConfigOverride, r.xdsConfig.VirtualHost.HTTPFilterConfigOverride)

And not doing so in the case where the route points to a cluster_specifier_plugin. See:

grpc-go/internal/xds/resolver/xds_resolver.go

Line 365 in c7ec4d9

if rt.ClusterSpecifierPlugin != "" {

This needs to be fixed. Interceptor chain needs to be build for all cluster types. As part of this change, we also need to add a test to ensure that filters work properly for routes that match to cluster_specifier_plugins.