Flagutil: utility for json file from directory or url and unmarshal to struct (#10333)

* initial commit

* adding first testcase wip

* fixing test

* adding more unit tests

* adding bazel file

* adding more unit tests and file checks

* addressing comments

* refactoring based on comments

* added bazel

* fixing build

validator/node/BUILD.bazel

@@ -4,6 +4,7 @@ go_test(
     name = "go_default_test",
     size = "small",
     srcs = ["node_test.go"],
+    data = glob(["testdata/**"]),
     embed = [":go_default_library"],
     deps = [
         "//cmd/validator/flags:go_default_library",