Define advanced dependencies in a bundle

[carolynvs]

This task covers everything needed to define and build a bundle that uses the advanced dependencies extension (deps v2).

• Add the new fields to the dependencies section of porter.yaml
• Provide a method on the manifest to determine which version of dependencies are in-use. Don't reach into the manifest and just go by a sentinel field, use this method instead so that we can maintain this going forward and be consistent.
• Convert porter.yaml to a bundle.json where the advanced dependencies features are included in a new cnab extension (under the porter namespace until we formalize the deps v2 spec upstream)
• Wiring between dependencies is converted from the shorthand porter format, e.g. bundle.dependencies.DEP.outputs.OUTPUT into the format used by our proposed deps v2 cnab extension. This feature has been removed, the templating is used without modification in our dependencies custom extension.
• Document the v2 deps extension format (which will eventually be submitted to CNAB after we have vetted it). Since we aren't submitting a PR upstream, the extension schema should be documented in a branch in the cnab-spec repository instead.

Notes

• Building a bundle that uses deps v2 should generate a bundle that has the deps v2 extension populated in the bundle.json.
• When the feature flag is disabled, but the v2 deps features are used, build should warn that v2 was used but is disabled. The resulting bundle should not have v2 extension data in the resulting bundle.