Support running Blueprint packages containing a Blueprint and Assets used by the Blueprint

What if we support running Blueprints from a Blueprint+Assets package where the Blueprint has access to assets included with the package zip?

More specifically:

• Create a zip containing a blueprint at a well-known path like META-INF/blueprint.json (might as well use a special dir name like epub and Java .jar's use to avoid path conflicts).
• Allow assets to be present anywhere else in the zip.
• Allow blueprint steps to reference those assets.

We could use this kind of packaging for Blueprints in general because it can be awkward to encode binary data or larger things entirely within a JSON file. Being , but one specific use case would be site exports or snapshots. Exports could be represented by WXR files, a SQLite database, or anything else, and the packaged blueprint would have steps for consuming the export.

For example, a package containing a blueprint, a WXR, and images under wp-content/uploads could have blueprint steps to do things like:

• Copy uploads into place
• Import WXR (possibly after adjusting media URLs if needed)

Perhaps Snapshots Support #9 could also be implemented using this kind of package.

What do you think?

@adamziel @bgrgicak @zaerl @akirk