[JS] Support custom_metadata

[asfimport]

Support field-level and schema-level custom metadata

Reporter: Brian Hulette / @TheNeuralBit
Assignee: Paul Taylor / @trxcllnt

Related issues:

• [JS] Implement JavaScript-compatible implementation (Is contained by)

_{Note: This issue was originally created as ARROW-1032. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Resolved by PR 527af63