[REQ] store and expose via `getApiClient()` the `ApiClient` used to create a library:native Api object

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

most (the base + httpclient, google-api, jersey2/3, okhttp, etc) Api objects expose the ApiClient used to create them via a public ApiClient getApiClient() method. library:native does not which makes it impossible to create additional Api objects from an Api object without using the Configuration.getDefaultApiClient() singleton (which may or may not be what was used to create a particular Api object).

Describe the solution you'd like

add a public ApiClient getApiClient() method to library:native's api.mustache.

Describe alternatives you've considered

passing the ApiClient around and forcing methods to create their own Api wrappers or defaulting to the global singleton Configuration.defaultApiClient.

Additional context

happy to implement this myself. PR forthcoming.

Read the library:native api.mustache template and compare how other libraries expose their ApiClient. Add the requested public accessor there, then verify that generated native Api objects expose the client used to create them.