Generate alternative AMP JS URLs

For performance reasons, it's helpful to be able to generate either AMP JS URLs that are either self-hosted (e.g. `https://example.com/v0.js`; avoids additional host lookup and HTTPS negotiation), or on "versioned" URLs (e.g. `https://cdn.ampproject.org/rtv/011905021827420/v0.js`; better chance of browser cache re-use).

This parallels the `ampRuntimeVersion` and `ampUrlPrefix` configurations of https://github.com/ampproject/amp-toolbox/tree/master/packages/optimizer.

(A corresponding change to the validator is also required.)

/cc @sebastianbenz

Start by tracing the existing AMP JS URL generation in amppackager and compare the requested options with ampRuntimeVersion and ampUrlPrefix in amp-toolbox's optimizer package. Check how the validator is connected; done means supporting self-hosted and versioned URLs together with the corresponding validator change.

Written by the indexing model from the issue text.