Add support for referencing multiple sources within an individual Bundle resource

[timflannagan]

Goal: update the Bundle API and add support for Bundles that reference multiple source types that provisioners can unpack and aggregate into a single source.

See the e2e strawman for more information.

[timflannagan]

Note: this seems like it would introduce additional complexity in provisioner implementations, but seems reasonable given PodSpec's support multiple container references? In any case, moving this to the backlog for now until we get a concrete use case, or we feel like this is worth pursuing.