[Vector][ExtensionType] ComplexCopier requires that extension types' holder readers implement getField

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

https://github.com/apache/arrow-java/pull/892 made efforts to streamline and simplify extension readers and writers. In this, `ComplexCopier.copy` relies on `FieldReader.getField` being defined for extension types.

Usages of `ComplexCopier.copy` with readers backed by vector, e.g., `UuidReaderImpl` and `VariantReaderImpl` that do have `getField` implementation, should be OK (as there is concrete notion of Field). However, if holder readers, namely, `NullableUuidHolderReaderImpl` or `NullableVariantHolderReaderImpl` are used this results in an NPE.

Having said that `getField` method for holder readers appears to be a misnomer as we do not have a notion of a field for the holders. This can be seen from the absence of `getField` overrides for primitive types.

Beginne damit, ComplexCopier.copy zu lesen und die getField-Implementierungen in UuidReaderImpl und VariantReaderImpl mit NullableUuidHolderReaderImpl und NullableVariantHolderReaderImpl zu vergleichen. Reproduziere die NPE mit einem holder-basierten Extension-Reader und überprüfe anschließend, dass das Kopieren keine unpassende getField-Implementierung für Holder mehr erfordert.

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