feat: add method for mode discovery

[aaqilniz]

This PR and the related PRs at loopback-connector & loopback-connector-mysql will enable cli's discoverer to add properties and their options based on the database's strict mode.

Checklist

• [x] Sign off your commits with DCO (Developer Certificate of Origin)
• [ ] 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 8599495495

Details

• 12 of 13 (92.31%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 84.727%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/datasource.js	12	13	92.31%
<!--	Total:	12	13	92.31%	-->

Files with Coverage Reduction	New Missed Lines	%
lib/datasource.js	5	72.29%
<!--	Total:	5		-->

Totals
Change from base Build 8594443975:	-0.04%
Covered Lines:	7279
Relevant Lines:	8283

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💛 - Coveralls