As part of this ticket, implement the aggregation expression for $encStrContains.

As outlined in the design , ExpressionEncStrContains will inherit from the base class, ExpressionEncTextSearch.

This goal of this task :

Implement the minimal requirements to allow the expression to be parsed by query analysis and the server rewrites
Implement parse & serialize.
Implement type checking/constraint checking when parsing the operands.

class ExpressionEncStrContains : public ExpressionEncTextSearch    { 
public: ...
};