[FR] Request pointer for CustomErrorHandler

**Is your feature request related to a problem? Please describe.**
I need to return different HTTP 404 response when different entry points are called.
For example APIs and pages should return different 404 page.

**Describe the solution you'd like**
```
virtual HttpAppFramework &setCustomErrorHandler(
std::function &&resp_generator) = 0;
```

**Describe alternatives you've considered**
N/A

**Additional context**
N/A