PERF: compute kurtosis for ExtensionArray using groupby method

[Alvaro-Kothe]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

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Uses group_kurt to compute the kurtosis for extended arrays. It increases performance in asv benchmark from 6 seconds to 25 milliseconds.

$ asv continuous -E virtualenv:3.13 upstream/main HEAD -b kurt --show-stderr

| Change   | Before [7bf66609] <main>   | After [a2f1de11] <perf/kurt>   |   Ratio | Benchmark (Parameter)                                                                    |
|----------|----------------------------|--------------------------------|---------|------------------------------------------------------------------------------------------|
| +        | 2.48±0.01ms                | 2.96±0.2ms                     |    1.19 | rolling.Methods.time_method('DataFrame', ('expanding', {}), 'float', 'kurt')             |
| +        | 3.14±0.01ms                | 3.67±0.4ms                     |    1.17 | rolling.Methods.time_method('DataFrame', ('rolling', {'window': 1000}), 'float', 'kurt') |
| +        | 3.71±0.4ms                 | 4.18±0.2ms                     |    1.13 | rolling.Methods.time_method('DataFrame', ('rolling', {'window': 1000}), 'int', 'kurt')   |
| +        | 2.95±0.03ms                | 3.31±0.2ms                     |    1.12 | stat_ops.FrameOps.time_op('kurt', 'float', 0)                                            |
| -        | 4.30±0.01ms                | 3.78±0.4ms                     |    0.88 | rolling.VariableWindowMethods.time_method('Series', '1d', 'float', 'kurt')               |
| -        | 6.89±0.03s                 | 25.2±3ms                       |    0    | stat_ops.FrameOps.time_op('kurt', 'Int64', 1)                                            |

[mroeschke]

Thanks @Alvaro-Kothe