[REQ] ability to provide an ApiClient factory for Configuration to use when creating a new defaultApiClient

[ronreynolds]

Is your feature request related to a problem? Please describe.

a mechanism for using code to customize the ApiClient lazily created by Configuration in getDefaultApiClient() would remove the need to overwrite the existing value with a custom ApiClient with auth headers, logging, and such via the request/response interceptors.

Describe the solution you'd like

the addition of a Configuration.setApiClientFactory(Supplier<ApiClient>) so that, on startup, and once auth header values have been established, all subsequent code can use Configuration.getDefaultApiClient() to retrieve the client created by this factory. also a call to setDefaultApiClient(null) will trigger a subsequent call to the factory on the next call to getDefaultApiClient() to support the use case where auth header values have changed or some other request/response interceptor behavior has changed (such as logging request and responses). obviously the factory must be invoked in a thread-safe way and only once per new-ApiClient need, which is cleanly supported by a VolatileReference MUCH better than a volatile ref. support for code currently using setDefaultApiClient() must continue to work also.

Describe alternatives you've considered

call setDefaultApiClient() after creating a customized ApiClient object that all subsequent code can retrieve via getDefaultApiClient().

Additional context

see #20738