support direct scheme on rpc resolver

rpcx/internal/resolver/directbuilder.go

@@ -0,0 +1,30 @@
+package resolver
+
+import (
+	"strings"
+
+	"google.golang.org/grpc/resolver"
+)
+
+type directBuilder struct{}
+
+func (d *directBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (
+	resolver.Resolver, error) {
+	var addrs []resolver.Address
+	endpoints := strings.Split(target.Endpoint, EndpointSep)
+
+	for _, val := range subset(endpoints, subsetSize) {
+		addrs = append(addrs, resolver.Address{
+			Addr: val,
+		})
+	}
+	cc.UpdateState(resolver.State{
+		Addresses: addrs,
+	})
+
+	return &nopResolver{cc: cc}, nil
+}
+
+func (d *directBuilder) Scheme() string {
+	return DirectScheme
+}