Changes in register entities API's

[sleipnir]

This PR changes the way entities are registered. Examples:

Before

fun main() {
    cloudstate {
        host = "0.0.0.0"
        port = 8080
        loglevel = "INFO"

        registerEventsourcedEntity {
            entityService = ShoppingCartEntity::class.java
            ....
        }
    }.start()
            .toCompletableFuture()
            .get()
}

After

fun main() {
    cloudstate {
        config {
            host = "0.0.0.0"
            port = 8080
            loglevel = "INFO"
        }

        eventsourced {
            entityService = ShoppingCartEntity::class
            ....
        }
    }.start()
            .toCompletableFuture()
            .get()
}

[sleipnir]

Ideally, this PR should be evaluated after this other one: https://github.com/cloudstateio/kotlin-support/pull/4

[sleipnir]

I'm closing this one because the next PR resolves an existing conflict with the master branch