add warm up trigger

[kaibocai]

add warm up trigger with detailed documents at here

test sample:

public class TestFunction {
    @FunctionName("Warmup")
    public void warmup( @WarmupTrigger Object warmupContext, ExecutionContext context) {
        context.getLogger().info("Function App instance is warm up");
    }
}

test results:

[shreyas-gopalakrishna]

LGTM!