Support workspace_status_rule in addition to workspace_status_command

### Description of the problem / feature request:

`--workspace_status_command` can be used to gain e.g. repository information - a git commit, UTC timestamp, ...

We would much prefer to have a `--workspace_status_rule` that is run before the regular build phase in the workspace directory.

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

The target called by `--workspace_status_command` is OS specific and there is no "nice" way to get around it. A colleague posted #5958 and we iterated with Bash/Powershell polyglot scripts and python, but even python seems to be phasing out as a requirement for running Bazel (which I consider a good thing, keep the requirements minimal).
Also, it's rather wild to get Python to write `\n` line endings under Windows...

We do not see a sound required common language we can use, here. Relying on Bazel itself would provide a solid foundation, though. This would probably require a "prebuild" phase. If that is too much, we can _probably_ also get by with a way load and run a starlark function.

But a rule is much more desirable and flexible than a command executed in a shell...

Aside: thank you so much for your work. We love Bazel!

Start by tracing the existing --workspace_status_command handling and how it runs in the workspace directory before the build phase. Define the rule interface and lifecycle needed to provide equivalent workspace status without an OS-specific command; done means the proposed rule works across platforms and its behavior is covered by appropriate tests and documentation.

Written by the indexing model from the issue text.