Add support for FileIO

It is registered in FileIO however neither `load` nor `fileio_load` is defined. Not sure if the following simple code is the correct way to do it.

```
fileio_load(file) = Arrow.Table(file.filename)
```

Start by locating the FileIO registration and checking where load and fileio_load are expected to be defined. Compare the proposed fileio_load(file) = Arrow.Table(file.filename) behavior with the existing Arrow.Table entry point. Done means FileIO can load supported files through both load and fileio_load, with relevant tests passing.

Written by the indexing model from the issue text.