Allow Starlark rules to observe the `--stamp` setting

### Description of the feature request:

As noted in [a comment on #1054](https://github.com/bazelbuild/bazel/issues/1054#issuecomment-219151351), Starlark rules can access `volatile-status.txt` as `ctx.version_file` and `stable-status.txt` as `ctx.info_file`, but there is no way for a Starlark rule to observe the `--stamp` flag setting to know if the rule should access the files or not.

### What underlying problem are you trying to solve with this feature?

I want to write Starlark rules that do not access the status files unless the user specifies the `--stamp` setting.

Because of issues like #10075 and #10177 it is important that unstamped builds are shielded from any access to the status files:
- #10177 breaks all caching when you access `stable-status.txt` even if you set your workspace status command to `/bin/true`.
- Working around #10075 forces us to use `no-remote-exec` on our stamped actions that access `volatile-status.txt`, which is a bottleneck for remote builds.

I tried using a custom build flag as a work-around to propagate my own version of the stamp setting, but it causes all outputs through transitions to change their output path and destroys all caching -- even on targets that don't depend on the new stamp-like setting.

### What operating system are you running Bazel on?

macOS and Linux

### What's the output of `bazel info release`?

```
release 3.0.0
```