[kotlin] Add OkHttpClient.Builder to ApiClient.

[ejithon]

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Description of the PR

#7872

Default constructor of OkHttpClient generates a Builder instance implicitly, but I need to customize it. For instance, set some interceptors by calling addInterceptor()

So I add an explicit Builder to ApiClient.

[wing328]

cc @jimschubert for review.

[jimschubert]

Sorry. a didn't see the cc from a week ago. The change looks good to me, although I didn't run to test it out.