Support kotlinx-coroutine suspend and Flow

[eitanliu]

Add features #383 Support kotlinx-coroutines suspend function.
Support kotlinx-coroutines Flow return type.

[kptfh]

Don't like idea of adding Kotlin dependency to core module. It will cause all pure Java users of this library to add this dependency to their projects

[eitanliu]

The Kotlin dependency uses the provided scope, which means it only participates in compilation, and the final product will not include the Kotlin dependency.