API/ENH: easier configurable NA values for parsing
xref #16079 (adding lowercase `n/a` to default missing values) - I have a case where I want to treat `'n/a'` as non-missing, but keep the rest of the default behavior.  It seems this is about the best I can do, which involves an internal value.
```python
from io import StringIO
data = '''a,b
1,2
3,n/a'''
pd.read_csv(StringIO(data), 
    na_values=pd.io.common._NA_VALUES.difference(['n/a']), 
    keep_default_na=False)
```
Not suggesting reverting #16079, but wonder if if there might be a better way to expose through some kind of option?  Initial idea was something like `pd.options.na_values` (set to the current `_NA_VALUES`), though might be issues using a mutable value as an option.   Also could expose something like `pd.io.DEFAULT_NA_VALUES`

