Standardize config for binary & controller

[lgadban]

Existing "settings" processing is explicitly done via env vars, ref: https://github.com/kgateway-dev/kgateway/blob/main/internal/kgateway/extensions2/settings/settings.go#L7-L12

This was intended to be as lightweight as possible while still providing a standardized approach.

The controller binary is now ran via cobra and contains explicit CLI flags as well.

We should consolidate and standardize on way to configure kgateway so we don't have different ways to accomplish the same thing.

This may lead us to refactor the setup package that the cmd kgateway main package calls to make it easier to plumb configuration from flags || envvars. ~ useful context from @timflannagan

We will also need to consider the helm API for these settings/config -- not all config should be exposed via Helm API IMO

Possible Options:

• Viper is fairly idiomatic and allows to define configuration in a single place and demotes the env-var specifics to an implementation detail.
• Use Viper but defer config to CM that is mounted in kgateway