Experiments/Runs, allow user-defined Run Group names

When creating an experiment's [Run](https://sagemaker.readthedocs.io/en/stable/experiments/sagemaker.experiments.html#sagemaker.experiments.Run) instance in order to track a training or processing job, the user should have an ability to specify a custom name for this run's [run_group_name](https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/experiments/run.py#L167) .

We've been using SageMaker Experiments previously and are currently migrating from the standalone `smexperiments` library to the new, intergated SDK. Previously we had an ability to define a `Trial`and effectively group a number of experiment/job runs under this Trial's name. Our understanding is that the new `run group` concept is serving the same purpose, yet for a standalone job run (pipelines might be different) it is not possible to specify a user-defined `run_group_name` while defining the experiment's `Run` context.