java: allow aliased types to have custom mappings

Created by: fabiokung

Fixes #2858. UPDATE: I was wrong and misundertood the code, this PR does not fix the issue, but https://github.com/OpenAPITools/openapi-generator/pull/4182#issuecomment-546092134 discusses an alternative on how to accomplish the same. I'd still consider the original issue to be a bug, it should be possible to define type mappings for any type definition in the spec.

This will allow kubernetes-client-java to switch to openapi-generator (kubernetes-client/java#709).

The kubernetes API swagger spec has types aliased to primitive types, e.g.:

    "intstr.IntOrString": {
      "description": "IntOrString is a type that can hold an int32 or a string.  When used in JSON or YAML marshalling and unmarshalling, it produces or consumes the inner type.  This allows you to have, for example, a JSON field that can accept a name or number.",
      "format": "int-or-string",
      "type": "string"
    },

However, these types need to be mapped to custom types in the Java client, so we can have custom serialization for them. The generator is being invoked with the following options:

  <type-mappings>intstr.IntOrString=IntOrString,resource.Quantity=Quantity</type-mappings>
  <import-mappings>IntOrString=io.kubernetes.client.custom.IntOrString,Quantity=io.kubernetes.client.custom.Quantity</import-mappings>

Without changes in this PR, type-mappings gets ignored, since IntOrString is first resolved to its alias (string), before mappings are evaluated.

cc @wing328 @brendandburns @yue9944882