feature: add custom runner support for time series models like ARIMA using BentoML

[jsamantaucd]

Feature request I have implemented and tested a feature that trains a time series model - ARIMA and serves it using BentoML's custom runner to make predictions.

Motivation I believe it would be a nice guide to someone exploring time series models with custom runner implementation for models like ARIMA.

Others: This feature was tested on tag v1.0.20

[sebasmos]

oh that's a great feature

[xianml]

It would be Great if it is based on bentoml >= 1.2

[jsamantaucd]

Sure @xianml, I will create this example with bentoml >= 1.2, thanks for suggesting.