Proposal: Switch from Retrofit2 to OkHttp3

[tokuhirom]

When the SDK was first developed, we utilized Retrofit2 to improve implementation efficiency. However, nowadays, our API definitions are automatically generated from openapi YAML files, and the use of Retrofit2 has become less relevant.

Furthermore, with the lack of new versions of Retrofit2 since 2020, it might be a good time for us to consider alternatives. In my opinion, switching to OkHttp3 could bring about improvements in both code readability and maintainability.

I propose that we migrate from Retrofit2 to OkHttp3 for our HTTP client. I look forward to hearing the thoughts of others on this proposal. Thank you.