[All] Provide Java equivalents of Kotlin coroutines/Flows

[EdricChan03]

See #8

Calling suspend functions is impossible from Java as the Continuation interface appears to expect a sealed class that has an internal Kotlin class

API suggestions

(Suggestions are tentative as of now)

For single-value suspend functions

We could expose methods that return one of the following:

Object	Source	Implementation
CompletableFuture	Java 8+	CoroutineScope#future
ListenableFuture	Guava	CoroutineScope#future
Single	RxJava 2.x	rxSingle
Single	RxJava 3.x	rxSingle

For methods that return a Flow

We could expose methods that return one of the following:

Object	Source	Implementation
Flow.Publisher	Java 9+	Flow.asPublisher
Flowable	RxJava 2.x	Flow.asFlowable
Flowable	RxJava 3.x	Flow.asFlowable