[okhttp-gson] Add support for OAuth access token retry

[Kiran-Sivakumar]

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Description of the PR

@bbdouglas @JFCote @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger
This adds support for OAuth access token retry logic to the OkHttp-gson Java library.

This implements a similar solution to the one used in the retrofit library: 69d956f.

Changes:

• ApiClient.mustache
  • ApiClient constructor added to allow for necessary access token retry information to be initialized (if user wants token retry support). This constructor is only generated if OAuth is being used.
  • Added getter for token end point (TokenRequestBuilder).
• OAuthOkHttpClient class generated under auth/ to perform access token retry requests.
• RetryingOAuth class generated under auth/ to perform access token retry logic; implements the okhttp Interceptor interface (clean and modular).
• Dependencies added for maven and gradle.
• Samples updated.

fix #1057

[wing328]

Looks good to me although I've not tested it locally

[wing328]

@Kiran-Sivakumar thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544