[FEATURE REQ] Support sync/async methods for each API

**Is your feature request related to a problem? Please describe.**
This is for track 2 migration. In track 1, using RxJava with mixed sync/async methods is fine. However, after converting to Spring Reactor, we have to provide both to ensure the users can have well-supported sync/async scenario when using our library.

**Describe the solution you'd like**
To add the missing async methods.

**Information Checklist**
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