Improve interface for InngestFunction creation

[KiKoS0]

Refactors the InngestFunction creation to be interface + annotations based. Given that we don't know what the most Java idiomatic approach for this interface, I kept the old representation of the InngestFunction (made it internal) in case we want to have different approaches to create them (e.g: Builder pattern)

An InngestFunction is now created by extending the interface:

Java

@FunctionConfig(id = "fn-sample", name = "Example Function")
@FunctionEventTrigger(event = "event-2")
@FunctionEventTrigger(event = "event-1")
public class FollowupFunction extends InngestFunction {
    @Override
    public String execute(@NotNull FunctionContext ctx, @NotNull Step step) {
        return "Success";
    }
}

Kotlin

@FunctionConfig(id = "fn-sample", name = "Example Function")
@FunctionEventTrigger(event = "event-2")
@FunctionEventTrigger(event = "event-1")
class FollowupFunction : InngestFunction() {
    override fun execute(ctx: FunctionContext, step: Step): String? {
        return "Success"
    }
}

[darwin67]

Looks great!!!