Lazy VariantServiceProvider

[olavopeixoto]

Currently, when you use IVariantServiceProvider to resolve a service type variant it instantiates all configured service variants prior to resolve the selected one.
Ideally we'd want to lazy instantiate the resolved service only.

My use case is, I've got several implementations for a given interface, each requires a unique set of configuration properties and dependencies. If I configure my app to always resolve to a single variant I don't want to have all settings and dependencies available for the other implementations that I'm not using.

At the moment all my implementations must be configured and they are all loaded into memory prior to resolve the instance.