[C++] Add support for writing MetadataVersion::V4-compatible IPC messages for compatibility with library versions <= 0.17.1

[asfimport]

While we need to increment the MetadataVersion, we should not strand old library versions since V4 is backward compatible with V5.

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [Format] Add V5 MetadataVersion (is a child of)
• [FlightRPC][Integration] Add support for setting metadata version for integration tests (is required by)

PRs and other links:

• GitHub Pull Request #35
• GitHub Pull Request #7664

_{Note: This issue was originally created as ARROW-9265. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Unions should not be allowed when writing v4 data, and probably not when reading either (unless we implement the required compatibility code).

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7664
#7664