Add Union Logical type

[asfimport]

Add a union type annotation for Group types that represent a Union rather than a struct.
Models like Avro or Arrow would make use of it.

Reporter: Julien Le Dem / @julienledem
Assignee: Julien Le Dem / @julienledem

Related issues:

• Add NULL type to Bring Parquet logical types to par with Arrow (blocks)

PRs and other links:

• GitHub Pull Request #44
• PR

_{Note: This issue was originally created as PARQUET-756. Please see the migration documentation for further details.}

[asfimport]

Julien Le Dem / @julienledem:
PR: #44