Local timestamps fields for Avro schema are not supported

Avro schema supports the creation of local timestampe fields:
```java

LocalTimestampMillis datetimeType = LogicalTypes.localTimestampMillis();
field = new Schema.Field(attrDesc.getAttributeName(), Schema.createUnion(datetimeType.addToSchema(Schema.create(Schema.Type.LONG), Schema.create(Schema.Type.NULL)), doc, null);
```
But since there is no support in the AvroSchemaConverter.convertLogicalType method, these fields end up as BIGINT (as reported in e.g. Apache Drill).

**Reporter**: [Stefan Ziegler](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=edi.gonzales)

_{**Note**: *This issue was originally created as [PARQUET-2315](https://issues.apache.org/jira/browse/PARQUET-2315). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

从 issue 中提到的入口 AvroSchemaConverter.convertLogicalType 开始，跟踪 localTimestampMillis 逻辑类型目前是如何转换的。通过检查本地时间戳字段是否表示为时间戳而不是 BIGINT 来确认这项更改，同时包括 issue 中描述的 nullable union 示例。

由索引模型根据 Issue 内容生成。