[Parquet] Variant Value field is optional

[sdf-jkl]

Describe the bug, including details regarding any error messages, version, and platform.

Consensus was reached in apache/parquet-format#591 that Variant value field is mandatory.

• Arrow-go should make the field mandatory for the writer.
• Reading invalid files should be allowed for backwards compatibility and ability to read out of spec - GH-590: Allow VARIANT value to be omitted parquet-format#591 (comment)

Component(s)

Parquet