Surface toolchain and platform information through Build Event

### Description of the feature request:

Toolchains and Platforms and the automatic resolution system could get quite confusing for users who are new to RBE.

It would be nice if we could surface the toolchains, platforms (host, exec, target), related constraint_{setting,value} being used, and the auto-resolution result through Build Event Stream. This way, Build Event Service providers could surface this information to end users in an easier-to-consume format. Users could then share the events with relevant build teams to help debug common issues.

### Which category does this issue belong to?

Core

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

Platforms and Toolchains matrixes are hard to learn and debug.

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

_No response_

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

_No response_

### 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` ?

_No response_

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

_No response_

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

I think we could start with an event proto message for automatic toolchain resolution.

Ultimately, I think detailed information (Providers) from CC, Java, and/or any Starlark toolchains would be super useful to have as dedicated events as well.

Start by reviewing the existing Build Event Stream schema and the automatic toolchain resolution flow. Identify how host, exec, and target platforms, constraint settings and values, and resolution results are represented. Done means the event scope and proto shape are defined, including whether provider details from C++, Java, and Starlark toolchains belong in dedicated events.

Written by the indexing model from the issue text.