[C++][Dataset] Add CsvFileFormat for CSV support

[asfimport]

This should be a minimal implementation that binds 1-1 file and ScanTask for now. Streaming optimizations can be done in ARROW-3410.

Reporter: Francois Saint-Jacques / @fsaintjacques
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #7033

_{Note: This issue was originally created as ARROW-7759. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
@pitrou do you think you could take a look at this?

[asfimport]

Antoine Pitrou / @pitrou:
Right, I'm adding it to my list of bugs :-)

[asfimport]

Antoine Pitrou / @pitrou:
I think this will require some nested parallelism support to avoid deadlocks (if e.g. the dataset scanner spawns a task that calls the CSV reader which itself calls TaskGroup::Finish).

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 7033
#7033