ENH: enable concat for nullable Int with other dtypes
Following-up on https://github.com/pandas-dev/pandas/pull/33607 which added the `ExtensionDtype._get_common_dtype` method to the EA interface to determine concat/append behaviour, we can now use this to refine the concat behaviour for nullable integer and boolean dtypes.

See comment here: https://github.com/pandas-dev/pandas/pull/33607#discussion_r410127707

For now, the PR only enabled concat of all IntegerDtype objects (any other combination will result in object dtype). But, we can expand this to the combination of IntegerDtype with numpy int dtype, Int with boolean, Int with float, etc

