feat(retrofit): add support for retrofit2 clients that handle errors by generating Spinnaker(Server|Http|Network)Exceptions

[Luthan95]

Global Error handling approach for changing from retrofit 1.9 to retrofit 2.x version
Creating Custom callAdaptor to handle the failure and throw Custom exception using retrofit2 response
This will be used throw the exceptions SpinnakerHttpException, SpinnakerNetworkException and SpinnakerServerException based on response status. So this will help in handling exception/error globally

References for ErrorHandler:
https://github.com/square/retrofit/blob/parent-2.8.1/samples/src/main/java/com/example/retrofit/ErrorHandlingAdapter.java
https://github.com/square/retrofit/blob/parent-2.8.1/retrofit/src/main/java/retrofit2/DefaultCallAdapterFactory.java

Retrofit2 client with error handler sample:-

Service svc = new Retrofit.Builder()
        .baseUrl("endpoint") // endpoint of service 
        .client(okHttpClient) // okhttp3 client with required interceptors and properties 
        .addCallAdapterFactory(ErrorHandlingExecutorCallAdapterFactory.getInstance())  // Call factory to handle error globally and to make use of custom spinnaker exceptions
        .addConverterFactory(JacksonConverterFactory.create(objectMapper))
        .build()
        .create(Service.class); 

Retrofit2 API sample:

@GET("/fetch/")
 Call<ResponseBody> find(@Query("q") String name);

Use of Retrofit2 client call sample:

By default execute method of retrofit2 call throws IOException, so to handle globally make use of Retrofit2SyncCall.execute(Call call) static method.

ResponseBody findResponse = Retrofit2SyncCall.execute(svc.find("name"))

Note:
New property which added to support the loglevel of retrofit2 client call, default to BASIC level

retrofit2:
  logLevel: BASIC // NONE,HEADERS,BODY

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• a37d5c6: added retrofit calladaptor to handle exception globally

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