Implement optional positional embedding in processor

[sahahner]

At the moment the latitude and longitude coordinates of the grid points are provided to the encoder only.

This PR implements the positional embedding for that transformer in the processor.

Related to this issue: https://github.com/ecmwf/aifs-dev/issues/10

Resolved points from anemoi-models PR

• shape of positional embedding flexible
• make use of positional embedding more flexible. use hydra instantiation

    Lat, Cos(lon), Sin(lon)
    Cos(lat), Sin(lat), Cos(lon), Sin(lon)

[HCookie]

This will need some updates to work with the recent pydantic changes, but apart from that LGTM.