[Java] Auto-generated OAuth example is out of date for many HTTP backends

Description

The code sample from the auto-generated Api Doc:

ApiClient defaultClient = Configuration.getDefaultApiClient();
// Configure OAuth2 access token for authorization: Client Credentials
OAuth Client Credentials = (OAuth) defaultClient.getAuthentication("Client Credentials");
Client Credentials.setAccessToken("YOUR ACCESS TOKEN");

no longer matches the ApiCilent implementation for most HTTP backends:

ApiClient defaultClient = new ApiClient("Client Credentials");
defaultClient.setAccessToken("YOUR ACCESS TOKEN");

and also seems to be invalid syntax.

Swagger-codegen version

2.2.1

Suggest a fix/enhancement

Use 2nd provided example for HTTP backends (feign, retrofit, retrofit2) that support TokenRequestBuilder for automatic request and renew of Access Token (and add documentation for that functionality too).

Start with modules/swagger-codegen/src/main/resources/Java/api_doc.mustache and compare its OAuth example with modules/swagger-codegen/src/main/resources/Java/libraries/feign/ApiClient.mustache. Check the corresponding feign, retrofit, and retrofit2 backends and their TokenRequestBuilder support. Done means the generated examples use valid, matching syntax and document automatic access-token requests and renewal where supported.