add subset algorithm

rpcx/internal/resolver/subset.go

@@ -0,0 +1,14 @@
+package resolver
+
+import "math/rand"
+
+func subset(set []string, sub int) []string {
+	rand.Shuffle(len(set), func(i, j int) {
+		set[i], set[j] = set[j], set[i]
+	})
+	if len(set) <= sub {
+		return set
+	} else {
+		return set[:sub]
+	}
+}