[PRE-RELEASE-0.4] Support URIs in artifact references

[blythed]

Example:

{
  "_base": "?test",
  "_builds": {
    "dill_lazy": {
      "_path": "superduper.components.datatype.get_serializer",
      "method": "dill",
      "encodable": "lazy_artifact"
    },
    "1c94cb0a8bbabb82c63be6ac52d3a1edf4fe5d79": {
      "_path": "superduper.components.datatype.LazyArtifact",
      "datatype": "?dill_lazy",
      "blob": "&:blob:s3://<path-to-artifact>"
    },
    "test": {
      "_path": "superduper.components.model.ObjectModel",
      "object": "?1c94cb0a8bbabb82c63be6ac52d3a1edf4fe5d79"
    }
  },
  "_files": {}
}

1. When Document.decode(component_json) is called, the artifact should be downloaded if it contains these references.
2. When a template is used to create a component, these artifacts should get downloaded when the template is used for the first time.

Support:

• s3
• http