Cross-cloud object_storage_object references

[goncalo-rodrigues]

Support for cross-cloud object storage is dependent on:

• Getting access to private resources from a different cloud: in azure this is done by a shared access signature; in aws there isn't any terraform support for it, but there is the concept of pre-signed urls (but not sure how to get one)
• Add an interface or use CloudDependentResource to represent a resource in a given cloud and add mHcl support for it
• Research how urls are passed around to understand if there is any way they are publicly accessible