[C++][Dataset] Support async streaming interface for getting fragments in Dataset

Add `GetFragmentsAsync()` and `GetFragmentsAsyncImpl()` functions to the generic `Dataset` interface, which allows to produce fragments in a streamed fashion.

This is one of the prerequisites for making `FileSystemDataset` to support lazy fragment processing, which, in turn, can be used to start scan operations without waiting for the entire dataset to be discovered.

To aid the transition process of moving to async implementation in `Dataset`/`AsyncScanner` code, a default implementation for `GetFragmentsAsyncImpl()` should be provided (yielding a VectorGenerator over the fragments vector, which is stored by every implementation of Dataset interface at the moment).