Add expr operator for Filter (#47)

* Add expr operator for Filter

https://www.mongodb.com/docs/manual/reference/operator/query/expr/

* Add test for Filter.expr

modules/kernel/src/main/scala/mongo4cats/operations/Filter.scala

@@ -321,6 +321,17 @@ object Filter extends AsJava {
   def where(javaScriptExpression: String): Filter =
     FilterBuilder(Filters.where(javaScriptExpression))
 
+  /** Creates a filter that matches all documents for which the given expression is true.
+    *
+    * @param expression
+    *   the aggregation expression
+    * @return
+    *   the filter
+    * @since 3.6
+    */
+  def expr[TExpression](expr: TExpression): Filter =
+    FilterBuilder(Filters.expr(expr))
+
   /** Creates a filter that matches all documents where the value of a field is an array that contains all the specified values.
     *
     * @param fieldName

modules/kernel/src/test/scala/mongo4cats/operations/FilterSpec.scala

@@ -81,6 +81,10 @@ class FilterSpec extends AnyWordSpec with Matchers {
       Filter.where("foo") isTheSameAs Filters.where("foo")
     }
 
+    "expr" in {
+      Filter.expr("$expr: { $gt: [ $spent , $budget ] }") isTheSameAs Filters.expr("$expr: { $gt: [ $spent , $budget ] }")
+    }
+
     "all" in {
       Filter.all("foo", "bar") isTheSameAs Filters.all("foo", 'b', 'a', 'r')
     }