[java][okhttp-gson-nextgen] better oneOf implementation (#11146)

* add validJsonObject method

* add check for null

* fix list model generation

* fix optional fields validation

* add tests

* fix variable naming

* update tests

* add fromJson in oneOf, add tests

* convert JSON to static

* remove trailing space

* add fromString methods to all models

* add toJson, fix anyOf template

* remove workarounds

* undo changes to tests

* skip file schema test

* add new file

samples/client/petstore/java/apache-httpclient/docs/ModelFile.md

@@ -0,0 +1,14 @@
+
+
+# ModelFile
+
+Must be named `File` for test.
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**sourceURI** | **String** | Test capitalization |  [optional]
+
+
+

samples/client/petstore/java/apache-httpclient/docs/ModelList.md

@@ -0,0 +1,13 @@
+
+
+# ModelList
+
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**_123list** | **String** |  |  [optional]
+
+
+