Make sagemaker an feature install

[milesgranger]

pip install stepfunctions[sagemaker]

Why? 🤔

Because sagemaker comes with some not-so-light dependencies. And in situations where the sagemaker module isn't used, we're taking up some valuable space.

ie. After installing we have things like scipy, numpy which aren't needed if one is not using the sagemaker module.

✦ ❯ du -h --max-depth=1  venv/lib/python3.7/site-packages/ | sort -hr
240M	venv/lib/python3.7/site-packages/
65M	venv/lib/python3.7/site-packages/scipy
56M	venv/lib/python3.7/site-packages/botocore
33M	venv/lib/python3.7/site-packages/numpy.libs
30M	venv/lib/python3.7/site-packages/scipy.libs
28M	venv/lib/python3.7/site-packages/numpy

[wong-a]

Resolving in favour of #184