Helper to get "root" error

[crepererum]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
It would be nice to get the typed "original" error, even when it is wrapped into a chain of ExternalError, ArrowError, Arc etc. The original error could be used to generate HTTP status codes in certain situations, e.g. "resource exhausted".

Describe the solution you'd like

impl DataFusionError {
    /// Get underlying error.
    ///
    /// This may be the same as `self`.
    pub fn find_root(&self) -> &Self {...}
}

Describe alternatives you've considered
String parsing? Sounds like bad code smell.

Additional context
See #4434 for improvements on the error generation. See #3940 for an application.

[crepererum]

FYI: I have an improved implementation of what we have in InfluxDB IOx. It just needs some clean ups and I'll file a PR for that.

[mingmwang]

There was a similar ticket I raised before.

#3410