Encode ID in extension metadata and verify when using

[alexlarsson]

In some cases you really need the extension to match the exact version of the main runtime. For instance in the debug info extension case. For these cases we should support recording some kind of id in the main runtime and the extension and if they don't match, avoid using an installed extension.

The obvious id to record would be the commit id, but there are cases when it changes post-creation, such as when we re-commit a build to the stable repo, so its not a great idea.