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).