Add support for $expr operator

[divyajnu08]

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[mp911de]

Thanks for submitting a pull request. We have dedicated operator classes (e.g. ArithmeticOperators) to build AggregationExpression which can be used with $project. For example:

Aggregation.addFields().addField("foo")
		.withValue(ArithmeticOperators.valueOf("otherField").stdDevPop()).build();

Aggregation.project().and(ArithmeticOperators.valueOf("otherField").stdDevPop()).as("otherFieldStdDevPop"));

Accepting AggregationExpression and field references in stages that do not accept these yet makes still sense. I suggest removing stdDevPop and stdDevSamp operators from this pull request as we have support for these already and updating this PR to accept AggregationExpression in stages that do not accept these yet.

[divyajnu08]

Hi Mark

I have removed the operators and added unit tests using ArithmeticOperators.
I think all operators support this operator.
Also kindly check the newly added $expr operator and changes in match operator if its correct.

Thanks

[mp911de]

Thank you for your contribution. That's merged and polished now. We need to follow up with #3587 to enable a better usage of $expr with our Query infrastructure.