fix(sdk): fix platform instance generation in the sdk

[hsheth2]

Also adds tests for the container entity.

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

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
metadata-ingestion/src/datahub/sdk/container.py	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (66.66%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Files with missing lines	Coverage Δ
metadata-ingestion/src/datahub/sdk/_shared.py	86.58% <100.00%> (+0.16%)	⬆️
metadata-ingestion/src/datahub/sdk/container.py	92.70% <0.00%> (+21.87%)	⬆️

... and 4 files with indirect coverage changes

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