[Java] Inconsistent logic for type IDs in Union vectors

The current logic for calculating the type IDs in UnionVector#getField and DenseUnionVector#getField is:
1. DenseUnionVector uses an increasing counter 
1. UnionVector uses the ordinal of the type enum
1. Both completely ignore the type IDs provided at construction as part of fieldType (if provided)

We encountered this inconsistency while testing a direct roundtrip of a union vector between pyarrow and Java with the C Data Interface ('direct' here means without using VectorSchemaRoot/RecordBatch). The identifiers for the type IDs differ after completing a roundtrip. 

 

**Reporter**: [Roee Shlomo](https://issues.apache.org/jira/browse/ARROW-14271) / @roee88

_{**Note**: *This issue was originally created as [ARROW-14271](https://issues.apache.org/jira/browse/ARROW-14271). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

UnionVector#getField と DenseUnionVector#getField から始め、構築時に fieldType とそれに指定された型 ID がどのように処理されるかを追跡します。pyarrow から Java への直接的な C Data Interface ラウンドトリップを再現し、ラウンドトリップ後も両方のベクター型が一貫した型 ID を保持することを確認します。

索引モデルが issue の本文から書いたものです。