Migrate sheets reading to use serde

We have a bunch of structs which we fill from Google Sheets, one per row, e.g.

https://github.com/CodeYourFuture/trainee-tracker/blob/aa0d5787598ab8191f6429b6858f3b62dd2a6f9f/src/prs.rs#L217-L224

We currently do this in an annoyingly manual and inflexible way.

For this particular struct, we populate it here:

https://github.com/CodeYourFuture/trainee-tracker/blob/aa0d5787598ab8191f6429b6858f3b62dd2a6f9f/src/reviewer_staff_info.rs#L48-L79

We ignore the headings completely, so if someone changed the sheet, we would start reading incorrect values. Then we hard-code the field order and types of each cell.

In other places, we manually check headings, and just error if they're wrong: https://github.com/CodeYourFuture/trainee-tracker/blob/aa0d5787598ab8191f6429b6858f3b62dd2a6f9f/src/register.rs#L105-L160

We also handle partial rows poorly.

I would love instead for us to just use `serde`. We could derive `Deserialize` for each struct we populate, and use a `serde` adaptor to read the sheet into a `Vec`.

There is an existing [`serde_sheets` crate](https://crates.io/crates/serde_sheets) for doing this, but it has some issues. If it runs into issues with a row, it just prints an error to stdout and continues (https://docs.rs/serde_sheets/latest/src/serde_sheets/lib.rs.html#200).

Ideally we would either improve this crate, or write our own one. I'm imagining supporting a function along the lines of:

```rust
pub async fn read_all(
sheets: &mut Sheets,
document_id: &str,
tab_name: &str,
) -> SheetReadResult;

enum SheetReadResult {
AllOk(Vec),
SomeOk {
ok: Vec,
partial_records: Vec,
deserialization_error_records: Vec<(Record, serde::Error)>,
},
Err(SheetsError),
}

struct Record {
row_number: usize,
cells: Vec,
}
```

まず src/prs.rs、src/reviewer_staff_info.rs、src/register.rs のシート読み取りパスを比較し、次に issue でリンクされている serde_sheets の動作を調査します。serde_sheets を改善するのか、アダプターを構築するのかを、ヘッダー、部分的な行、デシリアライズエラーをどのように表現するかも含めて決定します。関連する struct を現在の手動フィールド処理なしでシートから読み取ることができ、エラーが出力されて無視されるのではなく返されることを完了条件とします。

索引モデルが issue の本文から書いたものです。