arrow

ARROW-2255: [C++][Developer][Integration] Serialize custom field/schema metadata

Note (Re C++ test): if generated json and a converted file differ (only) in the metadata of a field of a nested type, the `VALIDATE` step failed to register that they were not equivalent. This was due to a bug in `Field::Equals`.

Closes #6556 from bkietz/2255-Serialize-schema-and-fiel

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>