Visitor Pattern

Visitor Pattern is used in segcore for parse and execute Execution Plan.

1. Inside ${internal/core}/src/query/PlanNode.h, contains physical plan for vector search:
   1. VectorPlanNode vector search execution node
2. ${internal/core}/src/query/Expr.h contains physical plan for scalar expression:
   1. TermExpr support operation like col in [1, 2, 3]
   2. RangeExpr support constant compare with data column like a >= 5 1 < b < 2
   3. CompareExpr support compare with different columns, like a < b
   4. LogicalBinaryExpr support and/or
   5. LogicalUnaryExpr support not

Currently, under ${internal/core/src/query} directory, there are the following visitors:

1. ExecPlanNodeVistor physical plan executor only supports ANNS node for now