Cart i2: additional classes for inner blocks do not appear on the front-end

[ralucaStan]

Describe the bug

When adding a custom class in the editor for any Cart i2 inner block, the class is not added in the front-end

To reproduce

Steps to reproduce the behavior:

1. Go to the editor and add the Cart i2 block
2. Click on the Order summary inner block
3. On the block's setting sidebar click Advanced and add a my-order-summary class under Additional CSS class(es)
4. Go to the frontend and see that the class does not appear in the frontend
5. Go back to the editor and notice that the class my-order-summary is still there

Expected behavior

The class should appear in the front-end

[mikejolley]

This is related to #4867 and the same issue as in #4963