Feature/external specifications

Pushing this in response to https://github.com/valueflows/valueflows/pull/520

Mainly need to confirm that the logic for external specifications makes sense like this, or if we should have ExternalProcessSpecification and ExternalResourceSpecification distinct from ExternalSpecification. I don't think it will make much difference to implementors - they will likely store the external URIs internally in a way that disambiguates whether they are process or resource specs simply by the records they are attached to.