[Kotlin][Client] Added supporting Retrofit2(RxJava/RxJava2/Coroutines) (#5750)

* Added support Retrofit2(RxJava/RxJava2/Coroutines) to Kotlin client code generator

* Added generated samples for Retrofit2(RxJava/RxJava2/Coroutines) on Kotlin

* Fixed generating retrofit2 without Rx/Coroutines

* Fixed MultipartBody template, remove redundant space after MultipartBody.Part annotation

* Fix documentation diff.

* Fix generating build.gradle file for samples.
Add dependencies for rxJava/rxJava2 and rx retrofit's adapter.

* Update generated sample with rx dependencies

* Update generated sample with coroutines

* Update generated sample with RxJava 2 dependencies

* Update and refactoring scripts for sample generation

* Update generated sample code

* revert changes by mistake

* revert changes by mistake #2

* Fix return type for first RxJava

* Add RxJavaCallAdapterFactory to ApiClient scheme for RxJava 1/2

* Fix script loggin

* Update generated code for RxJava 1/2

* Fix kotlin.md documentation by script export_docs_generators.sh

* Update Kotlin samples project in pom.xml

* Revert "Update Kotlin samples project in pom.xml"

This reverts commit 9de4d0ba

* Fixed "" wrapping number types for annotation value
Fixed generating polymorphic interfaces for Gson, interface fields can't be marked with @SerializedName annotation, it's enough to mark it in child model.
Fixed instantiationTypes(array, list, map) for Kotlin generator

* Update Kotlin samples project after last fixes

* Update Kotlin samples project for kotlin-jvm-*

* Update Kotlin readme.md documentations

* Update Kotlin client sample changes

* Fixed encoding braces for generics model

* Update Kotlin client sample changes

* Update Kotlin client sample after merge with master

* Fixed adding empty braces for inherit from Map/Array

* Update sample model after last fix with empty braces

* Revert adding @SerializedName to Kotlin interface fields through @get:SerializedName as for Jackson

* Update Kotlin client samples with adding @get:SerializedName

samples/openapi3/client/petstore/kotlin-jvm-retrofit2-rx2/docs/MapTest.md

@@ -0,0 +1,20 @@
+
+# MapTest
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**mapMapOfString** | **kotlin.collections.Map<kotlin.String, kotlin.collections.Map<kotlin.String, kotlin.String>>** |  |  [optional]
+**mapOfEnumString** | [**inline**](#kotlin.collections.Map<kotlin.String, InnerEnum>) |  |  [optional]
+**directMap** | **kotlin.collections.Map<kotlin.String, kotlin.Boolean>** |  |  [optional]
+**indirectMap** | **kotlin.collections.Map<kotlin.String, kotlin.Boolean>** |  |  [optional]
+
+
+<a name="kotlin.collections.Map<kotlin.String, InnerEnum>"></a>
+## Enum: map_of_enum_string
+Name | Value
+---- | -----
+mapOfEnumString | UPPER, lower
+
+
+