Java client enhanced Configuration with ApiClient Supplier

[ronreynolds]

a backwards-compatible enhancement to Configuration that supports:

• providing an Supplier<ApiClient> as a factory so that when the ApiClient is created using code can alter it while still using Configuration.getDefaultApiClient(). this also means that all *Api no-arg ctors will also use it.
• thread-safe changes to defaultApiClient beyond the volatile added earlier

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[ronreynolds]

sample usage of new ApiClient factory:

  static {
    Configuration.setApiClientFactory(() -> createClientForShop(defaultShopId));
  }

  public static ApiClient createClientForShop(Long shopId) {
    ApiClient apiClient = new ApiClient();
    apiClient.setRequestInterceptor(builder -> prepRequest(builder, shopId));  // adds required auth headers; optional request logging
    apiClient.setResponseInterceptor(ApiClients::processResponse);  // adds optional response logging          
    return apiClient;
  }

this callback would also allow user-code to extend ApiClient behavior via sub-classing; unfortunately much of that is currently being invoked via static methods in java-native; to be addressed in a future PR.

[ronreynolds]

#20769

[ronreynolds]

NOTE
this PR does create an unintended but observable change in code behavior; if one calls Configuration.setDefaultApiClient(null) the previous code would return null from Configuration.getDefaultApiClient(). it will now create a new ApiClient via the apiClientFactory. baring a factory that returns null getDefaultApiClient() will NEVER return null.