fix for #804 + java interface discriminator mapping fix

[OsztosA]

FIX for #804
Added support of discriminator mapping for Java interfaces too.
The current template does not use the discriminator defined in the Openapi definition file.

The implemented solution generate from this definition:

    "discriminator": {
      "propertyName": "aCustomProperty",
      "mapping": {
        "typeA": "#/components/schemas/classA",
        "typeB": "#/components/schemas/classB",
        "typeC": "#/components/schemas/classC"
      }
    },

this interface

@JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.PROPERTY,
property = "aCustomProperty")
@JsonSubTypes({
@JsonSubTypes.Type(value = ClassA.class, name = "typeA"),
@JsonSubTypes.Type(value = ClassB.class, name = "typeB"),
@JsonSubTypes.Type(value = ClassC.class, name = "typeC")
})
public interface Item {

For more details see GeneratorResultTestJava.interfaceWithCustomDiscriminator test.

The CodegenModel in "swagger-codegen" also had to be changed, that PR must be merged first: getSubTypeName() in CodegenModel

[HugoMario]

Hey @OsztosA
Thanks a lot for this PR, can you please fix the import reverting back the changes you made on it in order to merge this?