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ARROW-8647: [C++][Python][Dataset] Allow partitioning fields to be inferred with dictionary type

A maximum cardinality for the inferred dictionary can be specified:

| max | result |
|---|---|
| 0 (default) | All string partition fields will be inferred as utf8 |
| None | All string partition fields will be inferred as dictionary(<smallest integer>, utf8) |
| 64 | If a field has 64 or fewer distinct partition values, it will be inferred as dictionary(int8, utf8) otherwise as utf8 |

Closes #7536 from bkietz/8647-dictionary-partition-fields

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>