Support multiple toolchains in subrules

### Description of the feature request:

Subrules currently only support a single toolchain at a time, which limits their utility.

I understand that under automatic exec groups (which `subrule`s require for toolchain support) support for multiple toolchains in ordinary rules relies on [declaring explicit exec groups](https://github.com/bazelbuild/bazel/issues/19981#issuecomment-1895166984), so I suppose the request really is an ask for exec group declaration support in `subrule`s.

### Which category does this issue belong to?

Rules API

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

We have a set of helpers with which custom "sync" test rules (diff generated files against source files, with a `bazel run`-able update subcommand). This uses a copy-file and copy-to-directory toolchain (both from `@aspect_bazel_lib`), meaning it cannot be rewritten into a `subrule`.

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

Linux

### What is the output of `bazel info release`?

release 8.6.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by reading the subrule and automatic exec group behavior in the Rules API, then review the explicit exec group discussion linked from issue #19981. Use the copy-file and copy-to-directory toolchains from @aspect_bazel_lib as the motivating case. Done means subrules can declare and use multiple toolchains through explicit exec groups.

Written by the indexing model from the issue text.