Provide an option for buffering by duration

I'm trying to convert legacy RxSwift code where I was using the [`buffer(timeSpan:count:scheduler:)`](https://docs.rxswift.org/protocols/observabletype#/s:7RxSwift14ObservableTypePAAE6buffer8timeSpan5count9schedulerAA0C0CySay7ElementQzGG8Dispatch0K12TimeIntervalO_SiAA09SchedulerD0_ptF) methods.
I've found the .buffer in this package, but unfortunately, the current policies are all based on a number of items, not on a time span. Is it planned?

The issue names no implementation files or tests. Start by locating the existing buffer APIs and their item-count policies, then review related tests and determine the intended duration semantics before proposing coverage for time-based buffering.

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