Provide callback to allow user defined key-value metadata merging strategy

When merging footers, Parquet doesn't know how to merge conflicting user defined key-value metadata entries, and simply throws. It would be better to provide callbacks to let users define metadata merging strategies.

For example, in Spark SQL, we store our own schema information in Parquet files as key-value metadata (similar to parquet-avro). While trying to add schema merging support for reading Parquet files with different but compatible schemas, `InitContext.getMergedKeyValueMetaData` throws because we have different Spark SQL schemas stored in different Parquet data files. Thus, we have to overwrite `ParquetInputFormat` and merge the schema within `getSplits`, which is kinda hacky and inconvenient.

**Reporter**: [Cheng Lian](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=lian+cheng) / @liancheng
#### Related issues:
- [Release parquet-mr 1.6.0](https://github.com/apache/parquet-java/issues/1426) (is blocked by)

_{**Note**: *This issue was originally created as [PARQUET-194](https://issues.apache.org/jira/browse/PARQUET-194). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by reading InitContext.getMergedKeyValueMetaData and the existing ParquetInputFormat usage described in the issue. Trace how conflicting user-defined key-value metadata currently causes the merge to throw, then define and document a callback-based strategy that callers can provide. Done means compatible metadata can be merged without overriding ParquetInputFormat, while the default behavior remains covered by tests.

Written by the indexing model from the issue text.