Serialize Month objects

I want to serialize `Month` and `Year` etc. They do not have a unique representation in terms of seconds, so I try to serialize them directly.

```jl
using Dates, Arrow
ArrowTypes.arrowname(::Type{Dates.Month}) = :Dates_Month
ArrowTypes.JuliaType(::Val{:Dates_Month}) = Dates.Month
Arrow.toarrow(Month(1))
MethodError: Cannot `convert` an object of type Month to an object of type Second
```

Start by running the provided Julia reproduction and inspect the Arrow.toarrow conversion path for Dates.Month, Dates.Year, and related types. Determine how these values should be represented without conversion to Second; done means the reported Month serialization error is resolved and the affected date types serialize successfully.

Written by the indexing model from the issue text.