Support for non-generic API

[waf]

Hi, we are writing an nlog target for parquet, where developers can write structured objects to a logs, and these structured objects are then serialized to parquet.

We're running into an issue because ParquetSerializer.SerializeAsync<T> requires the type parameter T to be known at compile time, and then generates the ParquetSchema based on typeof(T). Because we're writing a general NLog target, we accept these structured objects as type Object, so we don't have a compile time type.

Would you be receptive to a ParquetSerializer.SerializeAsync overload that is non-generic, but takes the parameter as a type? e.g. ParquetSerializer.SerializeAsync(IEnumerable<object> objectInstances, Type objectType) or similar. I'd be happy to work on a Pull Request.

EDIT: I've looked into this, and see that ParquetSerializer is mostly based on Expressions, so this might be pretty difficult...