[Flight] Add application metadata field to DoGet

[asfimport]

As proposed on the mailing list, we should add a field for application-specific metadata in DoGet payloads and expose this in the APIs. The current APIs are rather RecordBatch-oriented, though.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

Related issues:

• [Flight] Enable integration testing of batches with dictionaries (supercedes)

PRs and other links:

• GitHub Pull Request #4282

_{Note: This issue was originally created as ARROW-4626. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4282
#4282