expose installed service worker script contents

[alvarolm]

First of all, thanks for all the work !, I also really appreciate the time taken to read this issue.

I'm in need to check the service worker once downloaded to verify the consistency as per a client requirement, this feature seems easy to implement since all we need is to expose the service worker script contents, maybe it could be via the ServiceWorkerRegistration object.

This is the only feature lacking to perform a complete integrity check of all webpage resources, since we already have Subresource Integrity (SRI) and other features.

I'm not proposing a behavior modification, this would be a feature in favour of transparency.

It would be useful for sensitive applications that require integrity verification and depend on service worker features.