feat: fetch unique index information for models

[aaqilniz]

At this point, the models have no information on indexes. This PR fetches index information for models.

Related PR:

PR@loopback-connector-mysql

Checklist

• [x] Sign off your commits with DCO (Developer Certificate of Origin)
• [x] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [x] Commit messages are following our guidelines

[coveralls]

Pull Request Test Coverage Report for Build 12098384263

Details

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 84.718%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/datasource.js	5	7	71.43%
<!--	Total:	5	7	71.43%	-->

Totals
Change from base Build 12034634278:	0.003%
Covered Lines:	7284
Relevant Lines:	8290

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💛 - Coveralls