SeriesGroupBy.quantile doesn't work for nullable integers

[dsaxton]

import pandas as pd

df = pd.DataFrame(
    {"a": ["x", "x", "y", "y"], "b": pd.array([1, 2, 3, 4], dtype="Int64")}
)
df.groupby("a")["b"].quantile(0.5)

raises

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-1-ef27f953b945> in <module>
      4     {"a": ["x", "x", "y", "y"], "b": pd.array([1, 2, 3, 4], dtype="Int64")}
      5 )
----> 6 df.groupby("a")["b"].quantile(0.5)

~/opt/miniconda3/lib/python3.7/site-packages/pandas/core/groupby/groupby.py in quantile(self, q, interpolation)
   1911                 post_processing=post_processor,
   1912                 q=q,
-> 1913                 interpolation=interpolation,
   1914             )
   1915         else:

~/opt/miniconda3/lib/python3.7/site-packages/pandas/core/groupby/groupby.py in _get_cythonized_result(self, how, cython_dtype, aggregate, needs_values, needs_mask, needs_ngroups, result_is_index, pre_processing, post_processing, **kwargs)
   2289                 func = partial(func, ngroups)
   2290 
-> 2291             func(**kwargs)  # Call func to modify indexer values in place
   2292 
   2293             if result_is_index:

pandas/_libs/groupby.pyx in pandas._libs.groupby.__pyx_fused_cpdef()

TypeError: No matching signature found

cc @ghuname xref #33071