feature request: `values` parameter for attr.string_list

### Description of the problem / feature request:

`attr.string()` currently supports a `values` parameter giving the valid values for the attribute. I believe that `attr.string_list()` should have a similar option, to define which values are legal in the given list of strings.

e.g.

```
foo = rule(
impl = _foo_impl,
attrs = {
'my_list': attr.string_list(values=["a", "b", "c"]),
}
)

# in a BUILD file
foo(
my_list = ["d", "a", "b"], # error - d isn't a valid value, but a and b are fine
)
```

### Feature requests: what underlying problem are you trying to solve with this feature?

easier validation of rule inputs. I can hardcode the list of acceptable values into my rule but then it's a little more code and some work to generate a useful error message.

The issue names the attr.string and attr.string_list entry points but no source files or tests. Locate their implementations and existing validation tests first; done means string_list accepts a values parameter and reports invalid list elements while allowing valid ones.

Written by the indexing model from the issue text.