Pluggable key material store

Encryption key material can be stored either inside Parquet files, or outside (configurable). For outside storage, Parquet already has a pluggable interface for custom implementations, `FileKeyMaterialStore,` but no mechanism to load them (currently, one implementation is packaged in parquet-mr, and always loaded when outside storage is configured). We will provide a way to load custom implementations of the `FileKeyMaterialStore`.

**Reporter**: [Gidon Gershinsky](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=gershinsky) / @ggershinsky
**Assignee**: [Gidon Gershinsky](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=gershinsky) / @ggershinsky

_{**Note**: *This issue was originally created as [PARQUET-2053](https://issues.apache.org/jira/browse/PARQUET-2053). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}