[I] [C++][Compute] Support casting a smaller struct to a bigger one if extra fields are nullable [arrow]

[via GitHub]

Tom-Newton opened a new issue, #44555:
URL: https://github.com/apache/arrow/issues/44555

   ### Describe the enhancement requested
   
   Arrow can already cast to a schema with an extra column if that column is 
nullable but it can't do the same on structs. 
   
   More precisely I want the follow unittests to pass
   ```
   TEST(Cast, StructToBiggerStruct) {
     std::vector<std::string> field_names = {"a", "b"};
     std::shared_ptr<Array> a, b;
     a = ArrayFromJSON(int8(), "[1, 2]");
     b = ArrayFromJSON(int8(), "[3, 4]");
     ASSERT_OK_AND_ASSIGN(auto src, StructArray::Make({a, b}, field_names));
   
     const auto dest =
         arrow::struct_({std::make_shared<Field>("a", int8()),
                         std::make_shared<Field>("b", int8()),
                         std::make_shared<Field>("c", int8(), 
/*nullable=*/false)});
     const auto options = CastOptions::Safe(dest);
   
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         TypeError,
         ::testing::HasSubstr("struct fields don't match or are in the wrong 
order"),
         Cast(src, options));
   }
   
   TEST(Cast, StructToBiggerNullableStruct) {
     std::vector<std::string> field_names = {"a", "b"};
     std::shared_ptr<Array> a, b, c;
     a = ArrayFromJSON(int8(), "[1, 2]");
     b = ArrayFromJSON(int8(), "[3, 4]");
     ASSERT_OK_AND_ASSIGN(auto src, StructArray::Make({a, b}, field_names));
   
     const auto type_dest = arrow::struct_(
         {std::make_shared<Field>("a", int8()), std::make_shared<Field>("b", 
int8()),
          std::make_shared<Field>("c", int8(), /*nullable=*/true)});
     const auto options = CastOptions::Safe(type_dest);
   
     c = ArrayFromJSON(int8(), "[null, null]");
     ASSERT_OK_AND_ASSIGN(auto dest, StructArray::Make({a, b, c}, {"a", "b", 
"c"}));
     CheckCast(src, dest);
   }
   ```
   
   
   
   Motivation comes from https://github.com/delta-io/delta-rs/issues/1610 but I 
think the fix would require a change to arrow. 
   
   
   
   ### Component(s)
   
   C++


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