[REQ] [Java] [Maven] InputSpec download from url

Created by: Catscratch

Currently inputSpec must be a file located in the filesystem.

It would be nice to provide an uri like "file:///..." or "http://..." and the maven plugins decides whether it should download the file first or not.

Usually to wanna generate sources for a public service providing a swagger.json file somewhere.

Changes could happen here: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java#L383