feat: provide support for groupby

[aaqilniz]

This PR enables the loopback-connector to apply group by filter & add min(), max(), count(), avg() & sum() in the result.

Related PRs:

• PR@loopback-datasource-juggler
• PR@loopback-next

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
• [x] Commit messages are following our guidelines

[coveralls]

Pull Request Test Coverage Report for Build 7842838503

• -5 of 27 (81.48%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 59.862%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/sql.js	22	27	81.48%
<!--	Total:	22	27

Totals
Change from base Build 7836864604:	0.5%
Covered Lines:	734
Relevant Lines:	1159

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💛 - Coveralls