Add UnleashSubscriber to the Autoconfiguration

[lbsekr]

Describe the feature request

I want the auto-configure component of this starter to also inject/configure a UnleashSubscriber bean into the Unleash bean.

Background

I do not want to replicate the auto-configuration logic just to configure a UnleashSubscriber in the Unleash Bean.

Solution suggestions

You could append the current auto configuration logic and pass a UnleashSubscriber bean to the UnleashConfig.Builder. Using the default @ConditionalOnMissingBean solution to pass a NoOpSubscriber if the implementing project does not define a custom UnleashSubscriber.