String dtype: rename the storage options and add `na_value` keyword in `StringDtype()`
xref https://github.com/pandas-dev/pandas/issues/54792

Rename the storage options and add `na_value` keyword, i.e. from the PDEP: from current `storage="pyarrow_numpy"` to `storage="pyarrow", na_value=np.nan`

This does not yet deprecate the `"pyarrow_numpy"` string option or `"string[pyarrow_numpy]"` alias, as our tests are using that a lot (I would leave that for a follow-up PR to first update all tests and then deprecate it).


@mroeschke over to you ```bash
pandas/core/arrays/string_.py:123: error: Incompatible types in assignment (expression has type "tuple[str, str]", base class "StorageExtensionDtype" defined the type as "tuple[str]")  [assignment]
``` Thanks @jorisvandenbossche 