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

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

ExpressionEncStrStartsWith will be the first expression implemented, so ExpressionEncTextSearch must be implemented as part of this task.

class ExpressionEncTextSearch : public Expression {

public:  

const auto& getEncryptedPredicateEvaluator() const;

// Returns true if string was encrypted binary payload.
// Otherwise we  have plaintext (i.e we are running in queryanalysis).
bool hasBinaryPayload() const;
protected:

ExpressionEncTextSearch(ExpressionContext* const expCtx,
                          intrusive_ptr<Expression> input,
                          intrusive_ptr<Expression> text);
private:  

EncryptedPredicateEvaluatorV2 _evaluator;

}; 

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.
• The business logic for evaluate() is to be handled in future work. For now, we just need to store the EncryptedPredicateEvaluatorV2 in the base class, and expose it via a getter.

class ExpressionEncStrStartsWith : public ExpressionEncTextSearch {
public:    
Value evaluate(...) const final;    
Value serialize(...) const final;    
const char* getOpName() const;
static boost::intrusive_ptr<Expression> parse(...);

void acceptVisitor(ExpressionMutableVisitor* visitor) final;
void acceptVisitor(ExpressionConstVisitor* visitor) const final;
}; 

Constraint checking for ExpressionEncTextSearch:

For ExpressionEncTextSearch, there are only two possible cases for the underlying value of text.

1. text is a constant expression with value type “String” -> plaintext data constant: 
2. An ExpressionEncTextSearch is only expected to have plaintext data when running in query analysis. 
3. An ExpressionEncTextSearch operating on plaintext data must never call evaluate(). Doing so, would imply that the encrypted operation is being evaluated on plaintext, which is not supported.
4. Query analysis code does not evaluate expressions, so within query analysis code, ExpressionEncTextSearch will only be used to parse the query such that it can be rewritten by query analysis, replacing the constant value with a BinData placeholder.

1. text is a constant expression with value type “BinData” -> encrypted BinData payload constant:
2. An ExpressionEncTextSearch with a BinData payload is expected when the query is received by the server.
3. Expressions are evaluated by the server, in particular, if we are performing a collection scan.

On the server, we will assert that hasBinaryPayload()  is true when ExpressionEncTextSearch::evaluate() is called.