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ARROW-11260: [C++][Dataset] Don't require dictionaries when specifying explicit partition schema

The API here is a little different than before, but this allows you to give a partition schema with dictionary fields, without having to fill in the dictionaries themselves. I opted to do this as part of the factory to avoid having to let partitionings be 'half constructed', and because the factory has the necessary logic already. As a bonus, this also lets you now check the inferred and actual types against each other.

I also fixed a bug when using non-int32 dictionary indices: a cast was missing.

Closes #9677 from lidavidm/arrow-11260

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>