Add `cxx_standard` constraints

I have wanted to configure targets in such a way that I can pass `-stdc++17` as a default value but remove this flag if a consumer wants to use a different version. Using [--copt](https://bazel.build/reference/command-line-reference#flag--cxxopt) is insufficient as it leads to targets being un-buildable unless consumers specify this on the command line or in their `.bazelrc` files which can be confusing or not feasible.

Would it be reasonable to define some `cxx_standard` constraints somewhere within `rules_cc` that's backed by a string flag such that users can specify on the command line or via a transition what version to use but would otherwise allow maintainers to set meaningful defaults that don't hinder folks who want to use different versions?