ENH: Add support for min_count keyword for Resample and Groupby functions
- [x] closes #37768
- [x] closes #37821
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This would add support for the documented keyword ```min_count``. If we decide to move forward with this, we have to decide what to do in the following case:
```
df = DataFrame({"a": [1] * 3, "b": [np.nan] * 3})
result = df.groupby("a").func(min_count=0)
```

``first`` and ``last`` return ``0.0`` which is fine I think and consistent with ``sum``, but ``min`` returns ``inf`` while ``max`` returns ``-inf``.

Default parameter is set to ``1`` to avoid backwards incompatible changes. We should keep this at least until 2.0

If we decide to not implement ``min_count`` here, we should adjust the docs and the function signatures for the groupby functions.

cc @rhshadrach 