Add union API to BloomFilter interface

Sometimes, one may want to build a file-level bloom filter by union all row groups bloom filters so that to save some memory. Add a union API that could make it easy to use.

**Reporter**: [Junjie Chen](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=junjie) / @chenjunjiedada
**Assignee**: [Walid Gara](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=garawalid) / @garawalid
#### PRs and other links:
- [GitHub Pull Request #768](https://github.com/apache/parquet-mr/pull/768)
- [GitHub Pull Request #770](https://github.com/apache/parquet-mr/pull/770)

_{**Note**: *This issue was originally created as [PARQUET-1815](https://issues.apache.org/jira/browse/PARQUET-1815). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start with the BloomFilter interface and review GitHub Pull Requests #768 and #770, which are linked from the issue. Compare their proposed union API and determine the expected behavior for combining row-group Bloom filters; done means the interface offers an easy way to build a file-level filter.

Written by the indexing model from the issue text.