Add support for Spring's RestTemplate in Java client codegen (#5404)

* Add support for Spring's RestTemplate in Java client codegen

Resolves #1176

* Update all Java samples to include change for Jackson enums

Currently enums are deserialized from a String value with an
explicit @JsonCreator. However, they are not being serialized with
this String value. This change introduces the @JsonValue annotation
on the toString() method of enums, so they will be serialized as a
String with the correct value.

samples/client/petstore/java/resttemplate/docs/MapTest.md

@@ -0,0 +1,19 @@
+
+# MapTest
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**mapMapOfString** | [**Map<String, Map<String, String>>**](Map.md) |  |  [optional]
+**mapOfEnumString** | [**Map<String, InnerEnum>**](#Map<String, InnerEnum>) |  |  [optional]
+
+
+<a name="Map<String, InnerEnum>"></a>
+## Enum: Map&lt;String, InnerEnum&gt;
+Name | Value
+---- | -----
+UPPER | &quot;UPPER&quot;
+LOWER | &quot;lower&quot;
+
+
+