Feat: Add "All assignees" in Task team member dropdown (#1763)

* implemented all select option FilterDropdownEntitySearchSelect and enabled it for tasks page filter

* created new filter operand IsNotNull for make a select all qraphql query, added internal state for tracking isAllEntitySelected

* used filterCurrentlyEdited to track if isAllEntitySelected is selected

* fixed filter button icon SelectAll Icon

front/src/generated/graphql.tsx

@@ -3043,6 +3043,7 @@ export enum ViewFilterOperand {
   GreaterThan = 'GreaterThan',
   Is = 'Is',
   IsNot = 'IsNot',
+  IsNotNull = 'IsNotNull',
   LessThan = 'LessThan'
 }