Add support for `--repo_contents_cache` (#2119)

**What type of PR is this?**

Feature

**What package or component does this PR mostly affect?**

go_repository

**What does this PR do? Why is it needed?**

This allows `go_repository` (with suitable attrs to ensure
reproducibility) and the bootstrapped Gazelle binary to be shared across
workspaces and `bazel clean --expunge` invocations. Requires Bazel 8.3.0
or later.

Running `bazel clean --expunge; USE_BAZEL_VERSION=8.3.0rc3 bazel test
//...` in `tests/bcr/go_mod` with primed caches:
```
Before:
INFO: Elapsed time: 14.479s, Critical Path: 0.56s
After:
INFO: Elapsed time: 6.753s, Critical Path: 0.60s
```

**Which issues(s) does this PR fix?**

**Other notes for review**