BUG: 1D slices over extension types turn into N-dimensional slices over ExtensionArrays
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- [X] I have confirmed this bug exists on the latest version of pandas.

- [X] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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#### Code Sample, a copy-pastable example

```python
class LoggingStringArray(pd.arrays.StringArray):
    """Extend StringArray to log arguments to __getitem__"""
    def __getitem__(self, item):
        print(f"Argument to __getitem__ is '{item}'")
        return super().__getitem__(item)

df = pd.DataFrame({"col1": LoggingStringArray(np.array(["hello", "world"], dtype=object))})
_ = df.iloc[:1]
```

Expected output:
```
Argument to __getitem__ is 'slice(None, 1, None)'
```

Actual output:
```
Argument to __getitem__ is '(Ellipsis, slice(None, 1, None))'
```

#### Problem description

If a DataFrame contains extension types, the block manager turns a 1D slice in the form `DataFrame.iloc[slice(x, y, z)]` into an N-dimensional slice over the ExtensionArray of the form `__getitem__((Ellipsis, slice(x, y, z))` . This problem breaks the `__getitem__()` method on one of our extension types in [Text Extensions for Pandas](https://github.com/CODAIT/text-extensions-for-pandas).

The `__getitem__` methods in Pandas' built-in extension types happens to work when fed this kind of n-dimensional slice
--- they just ignore the `Ellipsis` in the first position of the tuple. But I don't think they're supposed to accept that type of input.

The root cause is the new method `Block.getitem_block_index()`, added during some performance optimizations introduced in pandas-dev/pandas#40353:

```python
    def getitem_block_index(self, slicer: slice) -> Block:
        """
        Perform __getitem__-like specialized to slicing along index.

        Assumes self.ndim == 2
        """
        # error: Invalid index type "Tuple[ellipsis, slice]" for
        # "Union[ndarray, ExtensionArray]"; expected type "Union[int, slice, ndarray]"
        new_values = self.values[..., slicer]  # type: ignore[index]
        return type(self)(new_values, self._mgr_locs, ndim=self.ndim)
```

This method assumes that the contents of the block are 2-dimensional. The `ExtensionBlock` class inherits this method from `EABackedBlock`, which inherits it from `Block`.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : f00ed8f47020034e752baf0250483053340971b0
python           : 3.7.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.5.0
Version          : Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.0
numpy            : 1.21.0
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.3
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.25.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 4.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.7.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
