How can I get the column types of a Julia `DataFrame`?

```
using DataFrames

df = DataFrame(a = 1:4, b = ["a", "b", "c", "d"])

4×2 DataFrame
 Row │ a      b      
     │ Int64  String 
─────┼───────────────
   1 │     1  a
   2 │     2  b
   3 │     3  c
   4 │     4  d

```