Support CF Aggregation Variables

Is your feature request related to a problem?

New and fresh from the 1.13 CF conventions released in December 2025 : the « Aggregation variable », a new syntax for creating ncml-like aggregation within valid files (mostly netCDF).

https://cfconventions.org/Data/cf-conventions/cf-conventions-1.13/cf-conventions.html#aggregation-variables

The idea is to explicitly map external variables (i.e. in another file) to section of a "virtual" variable in the main file.

Describe the solution you'd like

Would be nice if xarray was able to open these transparently, with the correct combination of combine_by_coords and such.

Describe alternatives you've considered

As this is a I/O thing, I thought it would be best to be done here, instead of cf-xarray which concerns itself with more advance/high-level stuff. And making xr.open_dataset transparent to this seems a more user-friendly idea than having an extra function from an external package handle this.

Additional context

This new thing was a standalone extension to the CF conventions (under the name CFA) since 2023 (at least) and is already implemented by package cf-python.

Start by reading the CF 1.13 aggregation-variables specification linked in the issue, then inspect the existing xr.open_dataset and combine_by_coords behavior. Done means aggregation variables can be opened transparently with the correct combination of external data, without requiring a separate user-facing function.