[C++/Python] Provide access to "custom_metadata" Field attribute in IPC setting

[asfimport]

This will make it easier to write adapters for custom data types embedded in built-in Arrow types

Reporter: Wes McKinney / @wesm
Assignee: Phillip Cloud / @cpcloud

Related issues:

• Arrow 0.3 release (blocks)
• [Java] Provide API access to "custom_metadata" Field attribute in IPC setting (relates to)

_{Note: This issue was originally created as ARROW-483. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 588
#588