Cythonized GroupBy pct_change
- [ ] progress towards #19165
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This seemed like a logical follow up to #19673. ASV benchmarks below:

```bash
       before           after         ratio
     [1e4c50a5]       [c8285b5c]
+         170±7μs         239±20μs     1.41  groupby.GroupByMethods.time_method('int', 'min')
+        702±20μs         968±50μs     1.38  groupby.GroupByMethods.time_method('int', 'cumprod')
+         339±2μs          391±6μs     1.15  groupby.GroupByMethods.time_method('float', 'prod')
+      90.2±0.3μs          104±3μs     1.15  groupby.GroupByMethods.time_method('float', 'count')
-      617±0.09ms         917±40μs     0.00  groupby.GroupByMethods.time_method('int', 'pct_change')
-           1.07s       1.08±0.1ms     0.00  groupby.GroupByMethods.time_method('float', 'pct_change')
```