Support passing function to Appender
Possibly addresses issues discussed in #20074?  More generally, allows for less-verbose usage.
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/util/test_util.py`](https://github.com/jbrockmendel/pandas/blob/f8c4813dcbabbd7f797d3987af0a3d71e26560a0/pandas/tests/util/test_util.py) !

 - There are no PEP8 issues in the file [`pandas/util/_decorators.py`](https://github.com/jbrockmendel/pandas/blob/f8c4813dcbabbd7f797d3987af0a3d71e26560a0/pandas/util/_decorators.py) !

