[C++][Dataset] Expose more informational properties

[asfimport]

In thinking about what I'd want a useful print method for a Dataset in R to include, there are a few things that come to mind, and by skimming dataset.h, they're not available:

• How many Sources it has
• For a Source, what kind (local filesystem, other filesystem, etc.), base path (at least where we didn't provide a list of files), how many files, what file format

Reporter: Neal Richardson / @nealrichardson
Assignee: Francois Saint-Jacques / @fsaintjacques

PRs and other links:

• GitHub Pull Request #6374

_{Note: This issue was originally created as ARROW-7608. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
It would indeed be nice to have access to such information.
Additional information / inspection capabilities can be very useful for the purpose of debugging / understanding what is going on (eg being able to verify which file paths a dataset has discovered)

[asfimport]

Francois Saint-Jacques / @fsaintjacques:

• Dataset's source is found via Dataset::sources() returns vector

• Souce's kind Source::type_name() returns string

• Format's kind Format::type_name() returns string

  I'll add a method to FileSystemSource to return a list of file path.

[asfimport]

Neal Richardson / @nealrichardson:
Source's type_name doesn't appear to be useful: what is "tree"? https://github.com/apache/arrow/blob/master/cpp/src/arrow/dataset/dataset.h#L150

How do I get Format from Source?

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6374
#6374