Allow ReturnType to return Generics

[andrewwilsonnew]

Is your feature request related to a problem? Please describe.

I'd like to be able to change type mappings to return generics. So for example:

typeMapping.put("file", "Response<ByteString>");

However despite my {{{returnType}}} in the template we get: ResponseLessThanByteStringGreaterThan

Describe the solution you'd like

It would be great if {{{returnType}}} allowed generics

Describe alternatives you've considered

We currently do a manual work around and and use a kotlin typealias to fix this

Additional context

Waddyathink @wing328 @guiarn