Context: https://github.com/flutter/flutter/issues/131862

This PR injects a "realm" component to the storage base URL when the contents of the file `bin/internal/engine.realm` is non-empty.

As documented in the PR, when the realm is `flutter_archives_v2`, and `bin/internal/engine.version` contains the commit hash for a commit in a `flutter/engine` PR, then the artifacts pulled by the tool will be the artifacts built by the presubmit checks for the PR.

This works for everything but the following two cases:
1. Fuchsia artifacts are not uploaded to CIPD by the Fuchsia presubmit builds.
2. Web artifacts are not uploaded to gstatic by the web engine presubmit builds.

For (1), the flutter/flutter presubmit `fuchsia_precache` is driven by a shell script outside of the repo. It will fail when the `engine.version` and `engine.realm` don't point to a post-submit engine commit.

For (2), the flutter/flutter web presubmit tests that refer to artifacts in gstatic hang when the artifacts aren't found, so this PR skips them.