[Python] Change StructArray.field(..) to return "flattened" field?

Related to https://github.com/apache/arrow/issues/14946 on the C++ side, and this recently came up in https://github.com/apache/arrow/pull/14781#issuecomment-1339114243.

A StructArray has child arrays that make up its "fields", but in addition it can also have a top-level validity bitmap. So when accessing a field of a StructArray that has such top-level nulls, you can retrieve the "raw" child array or you can get the "logical" field array that combines the child array with the top-level bitmap.

To illustrate:

```
In [1]: arr = pa.StructArray.from_arrays([pa.array([5, 3, 4, 2, 1]), pa.array([1, 2, 3, 4, 5])], names=['a', 'b'], mask=pa.array([False, True, False, False, False]))

In [2]: arr.to_pandas()
Out[2]:
0 {'a': 5, 'b': 1}
1 None
2 {'a': 4, 'b': 3}
3 {'a': 2, 'b': 4}
4 {'a': 1, 'b': 5}
dtype: object

In [3]: arr.field('a')
Out[3]:

[
5,
3,
4,
2,
1
]

In [4]: arr.flatten()[0]
Out[4]:

[
5,
null,
4,
2,
1
]
```

Currently, the `field()` method on a StructArray gives you the raw child array, and there is a `flatten()` method that returns those "logical" field arrays for all the fields as a list of arrays.
We should have a method with which you can get the field array for a single field instead of having to use `flatten()`, and in https://github.com/apache/arrow/pull/14781, @amol- added a `_flattened_field` (private for now, but we needed it to get the correct values to sort by):

```
In [5]: arr._flattened_field('a')
Out[5]:

[
5,
null,
4,
2,
1
]
```

We could just make that a public method instead, however, some questions/concerns about this:

- I personally don't like the "flattened" term. I know we already use this in C++ as well (this basically just exposes the C++ `StructArray::GetFlattenedField`), but I don't find it very clear that it means this distinction.
- We could also change `field()` instead? I personally think this is what people typically will want when they currently call `field` (like @amol- was doing in the sort PR, to get the values of a certain field of the struct). The value in the raw child that is being masked by the top-level bitmap is kind of an implementation detail, and IMO a user should not necessarily get that so easily.
- If we would change `field()` to default to the "flattened" field, we need an alternative to access the raw child. We could add a keyword for this? (but what name?) Or a separate method like `child()`?

Start with the Python StructArray.field(), flatten(), and private _flattened_field() entry points described here, comparing raw-child and logical-field behavior when top-level validity is present. Resolve whether to expose a new public method, change field(), or add a raw-child alternative; done means the selected API and masking behavior are defined and implemented.

Written by the indexing model from the issue text.