Added op to Join operators (#216)

Author: javihern98

src/vtlengine/Operators/Join.py

@@ -9,6 +9,7 @@
 import pandas as pd
 
 from vtlengine.AST import BinOp
+from vtlengine.AST.Grammar.tokens import CROSS_JOIN, FULL_JOIN, INNER_JOIN, LEFT_JOIN
 from vtlengine.DataTypes import binary_implicit_promotion
 from vtlengine.Exceptions import SemanticError
 from vtlengine.Model import Component, Dataset, Role
@@ -276,6 +277,7 @@ def identifiers_validation(cls, operands: List[Dataset], using: Optional[List[st
 
 
 class InnerJoin(Join):
+    op = INNER_JOIN
     how = "inner"
 
     @classmethod
@@ -296,10 +298,12 @@ def generate_result_components(
 
 
 class LeftJoin(Join):
+    op = LEFT_JOIN
     how = "left"
 
 
 class FullJoin(Join):
+    op = FULL_JOIN
     how = "outer"
 
     @classmethod
@@ -320,6 +324,7 @@ def identifiers_validation(
 
 
 class CrossJoin(Join):
+    op = CROSS_JOIN
     how = "cross"
 
     @classmethod