Add extra group constants and validation to pkg/bootstrap/api.

This adds constants and validation for a new `auth-extra-groups` key on `bootstrap.kubernetes.io/token` secrets. This key allows a bootstrap token to authenticate to extra groups in addition to the `system:bootstrappers` group.

Extra groups are always applied in addition to the `system:bootstrappers` group, must begin with a `system:bootstrappers:` prefix, are limited in length, and are limited to a restricted set of characters (alphanumeric, colons, and dashes without a trailing colon/dash).

pkg/bootstrap/api/BUILD

@@ -3,12 +3,14 @@ package(default_visibility = ["//visibility:public"])
 load(
     "@io_bazel_rules_go//go:def.bzl",
     "go_library",
+    "go_test",
 )
 
 go_library(
     name = "go_default_library",
     srcs = [
         "doc.go",
+        "helpers.go",
         "types.go",
     ],
     deps = ["//vendor/k8s.io/api/core/v1:go_default_library"],
@@ -26,3 +28,9 @@ filegroup(
     srcs = [":package-srcs"],
     tags = ["automanaged"],
 )
+
+go_test(
+    name = "go_default_test",
+    srcs = ["helpers_test.go"],
+    library = ":go_default_library",
+)