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.*}

Start with the FileKeyMaterialStore interface and the existing implementation packaged in parquet-mr, then trace how outside key material is configured and loaded. Done means custom FileKeyMaterialStore implementations can be selected and loaded instead of always using the packaged implementation.

Written by the indexing model from the issue text.