Apply code-gen fix for union fields

[knottnt]

Issue #, if available: 2651

Description of changes:

• Rebuild with WIP code-gen fix for union fields
• Add .gitignore file

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knottnt

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[knottnt]

/test all

[ack-prow]

@knottnt: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
emrcontainers-verify-attribution	5d88b89	link	false	/test emrcontainers-verify-attribution

Full PR test history. Your PR dashboard.

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