* Better code for building Retrofit
By removing the nullable `usedClient` passed to Retrofit, the code is compatible with newer Retrofit versions which don't allow null as client.
* Updated Kotlin retrofit2 samples ApiClient
* added rx3 support
* fix rx3 support
* generated samples
* updated samples
* update samples
* changed rxjava3 adapter to the one from squareup
* changed dependency of RxJava3CallAdapterFactory
Co-authored-by: William Cheng <wing328hk@gmail.com>