Add magic to fetch Graph Explorer URL when deployed in SageMaker

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**Graph Notebook Version (and Graph Database and Version used if applicable)**

**Is your feature request related to a problem? Please describe.**
When deployed in a Neptune Notebook instance, Graph Explorer is deployed alongside Graph Notebook/Jupyter as a container hosted on the same SageMaker Notebook instance. Graph Explorer is exposed using Jupyter Proxy and port 9250.

Format of a Graph Explorer URL:
`https://.notebook..sagemaker.aws/proxy/9250/explorer/`

**Describe the solution you'd like**
Suggest that we include a magic in Graph Notebook that can detect if the user is running on a SageMaker Notebook / Neptune Notebook instance with Graph Explorer installed and provide the URL (clickable) to browse to Graph Explorer.

**Additional context**
N/A