Convert ExpressionWithPlaceholder into a MatchExpression

Class ExpressionWithPlaceholder is a pair of a MatchExpression, and a string. This is very unfortunate as it prevents this class from being processed as part of a MatchExpression tree.

Specifically this makes it very hard to implement common storage for the children of all MatchExpression classes, which in turn prevents the implementation of SERVER-54740.

This task should implement ExpressionWithPlaceholder as a subclass of MatchExpression with a single class member - boost::optional<std::string> _placeholder;