define a new udf signature to separate metadata from data transformation

[jdries]

Separating metadata transformation allows process graph analysis to work better when UDF's perform more complex transformations.
One example was requested in this issue:
Open-EO/openeo-geopyspark-driver#412

Applies for instance to UDF's that:

• change band labels
• change spatial resolution
• change crs of spatial dimension
• change timestamps on temporal dimension

I requested some reviews as this is an extension of the UDF API. It's a non-breaking change, UDF's that work will continue to work.
@m-mohr I'll want to make this an agenda topic on one of the next dev meetings. Do we require PSC approval for changes to UDF API?

[LukeWeidenwalker]

At EODC we do not support UDFs right now and it's currently not on our roadmap, so nothing to comment on from my side! (maybe @christophreimer has something to add)

[m-mohr]

@jdries Added to the agenda for September, @clausmichele is leading it in Sept.

The PSC guideslines are not overly clear on this: "Anything that could cause backward compatibility issues in normative specifications (e.g., API or process specifications)." Is the UDF API a normative specification? :-) But I'd say it doesn't hurt to at least present the anticipated changes there. Next PSC meeting is in October.

Edit: Nevermind, it's a non-breaking change so can just be merged, I think.

[soxofaan]

Looks good to me.