Allow for more customization of Unleash client

[GFriedrich]

Describe the feature request

Allow customizing the entire Unleash client without the need of exposing all builder functionalities via Spring.

Background

I would like to use this starter library, but on top I would like to customize specific parts that are not configurable by properties.
E.g. setting the fallback loader class to fulfill certain needs.

Solution suggestions

Therefore I would suggest to have another interface one could implement to customize the builder that gets created in the UnleashAutoConfiguration. And before the builder creates the instance, one does iterate over all customizers that were defined in the Spring context and in the end return the final builder.

Next to this it would be great if one could entirely replace the Unleash instance but still let the UnleashProperties be created. So adding a @ConditionalOnMissingBean to the Unleash bean would be very helpful.