Add SageMaker PipelineSession mock

[stiebels]

Hi @bblommers

SageMaker Pipelines has been updating their interface between Steps and Processors/Estimators/Tuners/... . They're now deprecating the old way of passing an estimator/processor/tuner object to the processor argument of a Step in favor of passing the output of processor.run (estimator.fit, ...) to step_args of the corresponding Step class (e.g. see here or here).

Currently, this is not supported in moto and trying to mock a Step that uses this interface results in an error (can c/p stack trace here later if needed; would outline in the PR eventually anyway).

In order to start supporting this, we'll need to add a mock for PipelineSession as an instance thereof is required to be passed to the processor/estimator/....

Happy make a PR adding this soon.

[bblommers]

Thanks for raising this @stiebels - a PR would be very welcome. Let me know if you need any help!