Manages the retrieval of externally referenced structures. These are determined by the isExternal attribute of a maintainable artifact being set
to true, and the URI attribute containing the URI of the full artifact.
Resolves external references, where the 'externalReference' attribute is set to 'true'.
The external reference locations are expected to be given by a URI, and the URI is expected to point to the
location of a valid SDMX document containing the referenced structure.
External references can be of a different version to those that created the input StructureBeans.
structures - containing structures which may have the external reference attribute set to `true`
isSubstitute - if set to true, this will substitute the external reference beans for the real beans
isLenient - if set true, this will add error notations if errors occur
a StructureBeans containing only the externally referenced beans