Enforce validators can only use the correct pubkey type (#2739)

* Enforce validators can only use the correct pubkey type

* adapt to variable renames

* Address comments from #2636

* separate updating and validation logic

* update spec

* Add test case for TestStringSliceEqual, clarify slice copying code

* Address @ebuchman's comments

* Split up testing validator update execution, and its validation

libs/common/string.go

@@ -61,3 +61,16 @@ func ASCIITrim(s string) string {
 	}
 	return string(r)
 }
+
+// StringSliceEqual checks if string slices a and b are equal
+func StringSliceEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}