[C++] Enhance arrow::internal::call_traits to support function pointers (and maybe ...?)

We are relying on call_traits in Future::Then to help type check the callables passed in.  Adding support for function pointers would allow us to simplify code from
```java

fut.Then([](const T& val) { return SomeCbFun(val); });
```
to...
```java

fut.Then(SomeCbFun);
```

**Reporter**: [Weston Pace](https://issues.apache.org/jira/browse/ARROW-12655) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-12655](https://issues.apache.org/jira/browse/ARROW-12655). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}