Plugin structures for FormData and upload/download

[zijianhuang]

OpenApiClientGen is designed for strongly typed data, and supporting FormData and upload/download is out of the inherent scope.

Handling non-json data especially upload/download could hardly be having a universal design/implementation since such operations involve fault tolerance and overall UX design, for example, the download progress handling.

It may be more flexible to provide some sort of plugin structure, for contributors of this projects and application developer to write a concrete solution for handling such scenarios.

related to #54