[Go] Enhancement to enum support in Go API client and servers

Description

Currently, the Go API client and server only supports enum class, e.g.

https://github.com/swagger-api/swagger-codegen/blob/2.3.0/samples/client/petstore/go/go-petstore/enum_test.go

But not enum for property/parameter type (EnumString, EnumNumber, EnumInteger) in enum_test.go

We also want to add some test cases in https://github.com/swagger-api/swagger-codegen/tree/2.3.0/samples/client/petstore/go for enum.

Swagger-codegen version

2.3.0

Swagger declaration file content or url

If anyone has an opinion on supporting enum property/parameter type in Go API client/server, please reply to let us know.

If anyone wants to contribute the test cases, please reply to let us know as well. Thanks.