[Rust] [Parquet] List schema to Arrow parser misinterpreting child nullability

We currently do not propagate child nullability correctly when reading parquet files from Spark 3.0.1 (parquet-mr 1.10.1).

For example, the below taken from https://github.com/apache/parquet-format/blob/master/LogicalTypes.md is currently interpreted incorrectly:

// List<String> (list nullable, elements non-null) 
optional group my_list (LIST) {
    repeated group list { 
        required binary element (UTF8); 
    } 
}

The Arrow type should be:

Field::new(
    "my_list",
    DataType::List(
        box Field::new("element", DataType::Utf8, nullable: false),
    ),
    nullable: true
)

but we currently end up with 

Field::new(
   "my_list",
   DataType::List(
       box Field::new("list", DataType::Utf8, nullable: true),
   ),
   nullable: true
)

This doesn't seem to be an issue with the master branch as of opening this issue, so it might not be severe enough to try force into the 3.0.0 release.

I tested null and non-null Spark files, and was able to read them correctly. This becomes an issue with nested lists, which I'm working on.