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

[asfimport]

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).

Reporter: Pavel Solodovnikov / @ManManson
Assignee: Pavel Solodovnikov / @ManManson

PRs and other links:

• GitHub Pull Request #13804

_{Note: This issue was originally created as ARROW-17318. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 13804
#13804