Add new functionality (--async) in weather-dl to terminate tool after dataflow job launched.

[dabhicusp]

Let's add a new flag like --async to weather-dl that enables the tool to automatically terminate and exit gracefully after getting the specific keyword from the log such as JOB_IS_STARTED.

Presently, upon completion of the entire dataflow job, our program terminates, resulting in significant time consumption. However, with the above modifications, our program exits as soon as the data flow job is launched.