[parquet_derive] support decoding Option<T> for ParquetRecordReader

[andrew-rj]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I'm trying to have a single struct with optional fields that can be written and read as parquet. I want to have something like this:

#[derive(ParquetRecordWriter, ParquetRecordReader, Default)]
pub(crate) struct TabularIdentifier {
    id: u64,
    type: String,
    maybe_thing: Option<String>,
}

This however does not compile. I get a compiler error that looks like this:

error: proc-macro derive panicked
   --> src/bin/commands/generators/parquet/records.rs:146:31
    |
146 | #[derive(ParquetRecordWriter, ParquetRecordReader, Default)]
    |                               ^^^^^^^^^^^^^^^^^^^
    |
    = help: message: not implemented: Unsupported: Option(
                TypePath(
                    Type::Path {
                        qself: None,
                        path: Path {
                            leading_colon: None,
                            segments: [
                                PathSegment {
                                    ident: Ident {
                                        ident: "String",
                                        span: #0 bytes(90325..90331),
                                    },
                                    arguments: PathArguments::None,
                                },
                            ],
                        },
                    },
                ),
            )

Describe the solution you'd like
It would be really helpful to be able to encode and decode a single struct that has optional fields via parquet-derive.

Describe alternatives you've considered
An alternative might be a more manual process that reads out the parquet data and constructs the struct. That is a fair bit more manual than I would like.

[tustvold]

FWIW the arrow APIs support this and more complex nesting, and will also be orders of magnitude faster. There are various options for converting structs to/from arrow, e.g. https://crates.io/crates/arrow_convert or https://crates.io/crates/serde_arrow