Add support for Parquet schema merging

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11017

Add support for Parquet schema merging so that we can read data sets where some files have additional columns.

[alamb]

Comment from QP Hou(houqp) @ 2021-02-20T20:05:55.859+0000:

The Schema struct does have a try_merge method defined, should we just reuse that one?