Update NA repr
Closes https://github.com/pandas-dev/pandas/issues/30415

```python

In [2]: df = pd.DataFrame({"A": pd.array([1, 2, None])})

In [3]: df
Out[3]:
      A
0     1
1     2
2  <NA>

In [4]: df.A
Out[4]:
0       1
1       2
2    <NA>
Name: A, dtype: Int64

In [5]: df.A.array
Out[5]:
<IntegerArray>
[1, 2, <NA>]
Length: 3, dtype: Int64

```

I think adding color with ANSI codes / custom HTML formatting is still worth doing as in #30778, but this is an improvement for now.
I had to include `<NA>` in the list of default NA values for read_csv to get this working. Probably OK to do. Also maybe simplified ExtensionBlock.to_native_types, but may have broken something along the way. On board with this once green some doc-test failures, otherwise lgtm. I think you need to update the tests if a nullable integer / bool / string array appears in Series/DataFrame? (or does this not work yet)? It works. We have smoke tests in the base class. Will add some dedicated ones. Passing now. Merging in an hour. > I had to include <NA> in the list of default NA values for read_csv to get this working. 

Can you explain why? Pandas does not write such format to CSV files, so it shouldn't be needed for a roundtrip