ARROW-6836: [Format] add a custom_metadata:[KeyValue] field to the Footer table in File.fbs
ARROW-6836: [Format] add a custom_metadata:[KeyValue] field to the Footer table in File.fbs

This is ARROW-6836 ... a separate issue (ARROW-6837) pertains to accessing the new field.  Unsure whether these are separate issues and/or pull-requests.  In case they are separate, here is the format one.

Closes #5649 from jgm-ktg/ARROW-6836 and squashes the following commits:

d36f02483 <John Muehlhausen> add a custom_metadata: field to the Footer table in File.fbs

Authored-by: John Muehlhausen <ed333bbb6a9a544c64028b81fcf68833e0c5a1f9@kershnertrading.com>
Signed-off-by: Micah Kornfield <dbc64ab8a675880673029ed6b2cb1d084dfef0bb@gmail.com>

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