[parquet-protobuf] Pass descriptor to ProtoWriteSupport constructor

Currently, the ProtoWriteSupport class checks for Descriptor by calling `Protobufs.getMessageDescriptor` function which checks for descriptor in the classpath. There is no way to pass descriptor as an argument to the ProtoWriteSupport constructor.

In our approach to using parquet-mr library, we are using a descriptor that is not available in the classpath.
I will be happy to work on adding this support to the parquet-mr library.

**Reporter**: [Maulik Soneji](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=mauliksoneji)

_{**Note**: *This issue was originally created as [PARQUET-1885](https://issues.apache.org/jira/browse/PARQUET-1885). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by reading the ProtoWriteSupport constructor and its use of Protobufs.getMessageDescriptor. Trace how descriptors are currently discovered and identify the constructor behavior needed when a descriptor is supplied directly. Done means callers can provide a descriptor that is not available on the classpath.

Written by the indexing model from the issue text.