fix(ingest): support async_flag properly in ingestProposalBatch

[hsheth2]

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 0% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...data-ingestion/src/datahub/emitter/rest_emitter.py	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
...data-ingestion/src/datahub/emitter/rest_emitter.py	78.77% <0.00%> (ø)

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[shirshanka]

LGTM