feat: show index information in openapi specs

[aaqilniz]

With these two PRs at loopback-datasource-juggler & loopback-connector-mysql, we'll have the unique index information in the model file. While with this PR, we can also have that information available in openapi specs.

Checklist

• [x] DCO (Developer Certificate of Origin) signed in all commits
• [x] npm test passes on your machine
• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [ ] API Documentation in code was updated
• [ ] Documentation in /docs/site was updated
• [ ] Affected artifact templates in packages/cli were updated
• [ ] Affected example projects in examples/* were updated

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