feat(sdk) Add python sdk example for read-modify-write structuredProperties

[chriscollins3456]

Adds a Python SDK example for read-modify-write the structured properties aspect of a given asset. Here you can see an example of adding, removing, and editing properties for an asset.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[chriscollins3456]

we can add this as a linked example in under entity model docs ..e.g. https://github.com/datahub-project/datahub/blob/master/metadata-models/docs/entities/dataset.md

[jjoyce0510]

Nice! Do we want to link out to the example code from any docs?

This is really useful!