Add service provider api to get instances of various services

Added a unit test. Existing tests are passing. @sumashivaprasad @Amareshwari  pls review.
Another implementation is unlikely.
I have added it as a config entry just so that there is no direct dependency of grill-server module on grill-server-api module. Otherwise we either have to keep the factory class name as a constant in grill-server-api, which is still an indirect dependency.