As part of this ticket, implement TextSearchPredicate and add support for $encStrStartsWith. This sets up the foundation for performing the server rewrites for all our encrypted text search expressions.

As described in the design , TextSearchPredicate is the specialization of EncryptedPredicate that will handle all encrypted text search expressions.

class TextSearchPredicate : public EncryptedPredicate {
public:

std::vector<PrfBlock> generateTags(BSONValue payload) const final;

std::unique_ptr<Expression> rewriteToTagDisjunction(Expression* expr) const final;

std::unique_ptr<Expression> rewriteToRuntimeComparison(Expression* expr) const final;
}; 

End to end testing will not be possible until server security delivers the FLE2FindTextPayload, as such, this task will be verified via unit tests.

Goals for this task:

• Implement TextSearchPredicate, leaving a stub for TextSearchPredicate::generateTags(). generateTags() will be implemented in a follow-up item once FLE2FindTextPayload is delivered.
• Register TextSearchPredicate as the rewriter for ExpressionEncStrStartsWith
• Make sure match expression methods of TextSearchPredicate throw an error, since match expressions for $encStrStartsWith are not supported, only aggregate expressions versions are supported.

Testing:

• Take a look at the way unit testing was done for other predicates in query_rewriter_test.cpp. We will follow a similar approach, which should allow us to unit test everything except generateTags(). generateTags() will be tested as part of the end to end testing effort.
• Make sure we add unit tests which ensure we don't rewrite the expression when we force a collection scan (look at 
  EqualityPredicateCollScanRewriteTest).