Make sagemaker an optional dependency

I'd like to see sagemaker marked as an optional dependency, as it is not always required.

### Use Case

Not all users of this package are using the sagemaker dependency, and it adds a lot of bloat to overall installations by forcing users to have gcc and g++ to be able to install numpy and pandas.

### Proposed Solution

It's quite simple to actually do this.

In `setup.py` you'd have to change `install_requires` and add `extras_require`:

```python
setup(
# [...]
install_requires = [
"boto3>=1.14.38",
"pyyaml"
]
extras_require = {
"sagemaker": ["sagemaker>=2.1.0"],
"test": [] # [...]
}
# [...]
)
```

In addition to this, some extra documentation should probably be added to the README to make it easier for users to understand how to install the extra dependency (`pip install stepfunctions[sagemaker]`).

### Other

If this is implemented, the major version probably has to be bumped because it's a change in how the package is installed.

---

* [x] :wave: I may be able to implement this feature request
* [x] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

Commencez dans setup.py et examinez install_requires et extras_require, puis consultez le README pour les instructions d’installation. Vérifiez que l’installation par défaut ne nécessite plus sagemaker et que les utilisateurs peuvent installer la dépendance facultative avec la commande pip documentée ; évaluez l’impact sur la version majeure avant de la modifier.

Rédigé par le modèle d'indexation à partir du texte de l'issue.