[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.*}

Beginne mit UnionVector#getField und DenseUnionVector#getField und verfolge anschließend, wie fieldType und die dabei bereitgestellten Typ-IDs während der Konstruktion verarbeitet werden. Reproduziere den direkten pyarrow-to-Java C Data Interface-Roundtrip und überprüfe, dass beide Vektortypen nach dem Roundtrip konsistente Typ-IDs beibehalten.

Vom Indexierungsmodell aus dem Issue-Text verfasst.