add buffer(duration) function

[helmbold]

To make Kotlin Flow more suitable for a wide range of tasks, I suggest to add an buffer(Duration) function like in Reactor.

Description

Collect incoming values into multiple List buffers that will be emitted by the returned Flow every bufferingTimespan.

Signature

fun <T> Flux<T>.buffer(bufferingTimespan: Duration): Flow<List<T>>

Consider to also implement:

Flux<List<T>> buffer(Duration bufferingTimespan, Duration openBufferEvery)

[elizarov]

It looks like a duplicate of #1302. Please, take a look.

[helmbold]

@elizarov Yes, it is a duplicate.