[Python][Dataset] Passthrough schema to Factory.finish() in dataset() function

[asfimport]

This is already a very simple fix to allow manually specifying the schema, without exposing any other options (as described in ARROW-8221)

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Joris Van den Bossche / @jorisvandenbossche

PRs and other links:

• GitHub Pull Request #6788

_{Note: This issue was originally created as ARROW-8292. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6788
#6788