feat:add operator support for partial json update

[vaibhavbhalla2505]

This PR and related PR at loopback-connector-postgresql will enable support for partial json update

Fixes #770

Checklist

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

[samarpanB]

Please add a test case for this change. How does this change adds support for partial json update ?

[vaibhavbhalla2505]

Please add a test case for this change. How does this change adds support for partial json update ?

done!

[coveralls]

Pull Request Test Coverage Report for Build 20988051064

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 59.035%

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Totals
Change from base Build 20928679139:	0.1%
Covered Lines:	723
Relevant Lines:	1153

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💛 - Coveralls