MonthlyDates

[`MonthlyDates.jl`](https://github.com/matthieugomez/MonthlyDates.jl) defines a `MonthlyDate <: TimeType`, which corresponds to elapsed months.

Unfortunately, it does not work when writing arrow files
```julia
using Arrow, MonthlyDates
Arrow.write("~/test.arrow", (a = [MonthlyDate(1990, 1)],))
ERROR: TaskFailedException
Stacktrace:
[1] wait
@ ./task.jl:317 [inlined]
[2] write(io::IOStream, source::NamedTuple{(:a,), Tuple{Vector{MonthlyDate}}}, writetofile::Bool, largelists::Bool, compress::Nothing, denseunions::Bool, dictencode::Bool, dictencodenested::Bool, alignment::Int64, maxdepth::Int64, ntasks::Float64)
@ Arrow ~/.julia/packages/Arrow/ggVa0/src/write.jl:165
[3] #94
@ ~/.julia/packages/Arrow/ggVa0/src/write.jl:86 [inlined]
[4] open(::Arrow.var"#94#95"{Bool, Nothing, Bool, Bool, Bool, Int64, Int64, Float64, Bool, NamedTuple{(:a,), Tuple{Vector{MonthlyDate}}}}, ::String, ::Vararg{String, N} where N; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Base ./io.jl:330
[5] open
@ ./io.jl:328 [inlined]
[6] #write#93
@ ~/.julia/packages/Arrow/ggVa0/src/write.jl:85 [inlined]
[7] write(filename::String, tbl::NamedTuple{(:a,), Tuple{Vector{MonthlyDate}}})
@ Arrow ~/.julia/packages/Arrow/ggVa0/src/write.jl:85
[8] top-level scope
@ REPL[17]:1

nested task error: MethodError: Cannot `convert` an object of type Dates.Month to an object of type Dates.Second
Closest candidates are:
convert(::Type{P}, ::Arrow.Duration{U}) where {P<:Dates.Period, U} at /Users/matthieugomez/.julia/packages/Arrow/ggVa0/src/eltypes.jl:357
convert(::Type{Dates.Second}, ::Dates.Minute) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Dates/src/periods.jl:417
convert(::Type{Dates.Second}, ::Dates.Hour) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Dates/src/periods.jl:417
```
Would it be possible for Arrow to accept `MonthlyDates`? Can I do anything on the `MonthlyDates.jl` side to improve compatibility?

Reproduce the failure with the Julia snippet, then inspect Arrow's write.jl and eltypes.jl at the stack-trace locations, along with MonthlyDates.jl. Done means Arrow.write accepts a vector of MonthlyDate values without the reported conversion error, with compatibility behavior confirmed by a regression test if the repository provides one.

Written by the indexing model from the issue text.