[Spring] Support MultiPartFile for file uploads

Somewhat similar to https://github.com/swagger-api/swagger-codegen-generators/issues/150 but for Spring specifically.

When using the Spring profile and a multipart service definition, I would expect to be able to influence the generated api Controller stub to use Spring's org.springframework.web.multipart.MultiPartFile.

The options I've tried and result:

• type: string, format: binary maps to a byte[]
• type: file, format: binary maps to a org.springframework.core.io.Resource (I would expect this to use MultiPartFile instead)

As an alternative, I extended the SpringCodegen and registered my own typeMapping.

typeMapping.put{"multiPartFile","MultiPartFile");
importMapping.put("MultiPartFile", "org.springframework.web.multipart.MultiPartFile");

With this I was able to get a controller stub generated using the MultiPartFile and support uploads. I'm pretty new to this product so I'm not sure if this is a best practice or not.

Start with the SpringCodegen extension and the typeMapping and importMapping examples in the issue; trace how multipart definitions become generated Spring controller parameter types. Check the existing Spring generator behavior and related generator tests, if present. Done means a Spring multipart definition can generate a controller stub using MultiPartFile with its correct import.