Add integer => date_time caster

Using the default :second unit:

defmodule TestContract do
  use Drops.Contract

  schema do
    %{required(:test) => from(:integer) |> type(:date_time)}
  end
end

TestContract.conform(%{test: 1695277470})
# {:ok, %{test: ~U[2023-09-21 06:24:30Z]}}

Providing time unit as an argument for the casting function:

defmodule TestContract do
  use Drops.Contract

  schema do
    %{required(:test) => from(:integer, [:millisecond]) |> type(:date_time)}
  end
end

TestContract.conform(%{test: 1695277723355})
# {:ok, %{test: ~U[2023-09-21 06:28:43.355Z]}}

Closes #10

solnic / drops

Add integer => date_time caster #22