Settings: actix.mode has no actix behaviour associated

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## Expected Behavior

I expect that all of the settings under the "actix" table correspond to some actix behaviour.

## Current Behavior

The config field "actix.mode" has no actix behaviour associated with it, and is ignored by `ApplySettings`.

## Possible Solution

IMO having a `mode` config is very helpful, being a very common concept, so I suggest it is moved into the `application` part of the config structure, or another config section being created such as "common" for config that actix-settings provides but doesnt have any implemented behaviour.

Another alternative is for actix-settings to use this `mode` value to provide different defaults for prod vs dev, and document this.

## Steps to Reproduce (for bugs)

## Context

## Your Environment