[feature] Extend the remote page to add custom page properties

**Is your feature request related to a problem? Please describe.**
I can't create any custom remote next pages with some additional page properties (so that the export model.json works as expected and exports these new fields) as I can't extend the PageImpl.class which is inside the internal package. Also, delegate can't be used. The only custom property available currently is the SPA url that's from the remote page itself.

**Describe the solution you'd like**
I'd like to be able to easily extend the behavior of the existing remote next page and to add additional page properties.

**Describe alternatives you've considered**
The only solution I see now is to have the internal package inside the codebase and then use it (hopefully I'm not missing something).

**Additional context**
It should be clear enough, if any additional context is required, let me know.

Start by locating PageImpl.class and the existing remote next page implementation in the internal package. Trace how custom page properties reach model.json and how the current SPA URL property is exposed. Done means consumers can extend the remote page behavior and export additional page fields without depending on the internal package or delegate.

Written by the indexing model from the issue text.