Fix/update table gsi

[joaquinservetto]

Issue #, if available:
aws-controllers-k8s/community#1920

Description of changes:
Fix error faced when creating a gsi for an existing table using already defined attributes.

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

[ack-prow]

Hi @joaquinservetto. Thanks for your PR.

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[a-hilaly]

/ok-to-test

[joaquinservetto]

Hey @a-hilaly, is there anything else missing to merge this?

[a-hilaly]

Thank you @joaquinservetto !
TIL You can reuse the same attributes to create more GSIs..
/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a-hilaly, joaquinservetto

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[a-hilaly]

@joaquinservetto Can we also add end2end tests to make sure that the table is in Synced stated?