feat: provide support for groupby

[aaqilniz]

This PR enables the loopback-datasource-juggler to add min(), max(), count(), avg() & sum() in the result.

Related PRs:

• PR@loopback-next
• PR@loopback-connector

Checklist

• [x] Sign off your commits with DCO (Developer Certificate of Origin)
• [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
• [x] Commit messages are following our guidelines

[coveralls]

Pull Request Test Coverage Report for Build 11316630714

Details

• 35 of 36 (97.22%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 84.737%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/dao.js	3	4	75.0%
<!--	Total:	35	36	97.22%	-->

Totals
Change from base Build 11297099147:	0.02%
Covered Lines:	7316
Relevant Lines:	8323

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💛 - Coveralls