API modules should be independent from server

[jdries]

Currently, at least 2 modules in API have imports that reference server:

openeo-udf/src/openeo_udf/api/udf_data.py

Line 12 in e2fc2c1

from openeo_udf.server.data_model.metadata_schema import MetadataModel

openeo-udf/src/openeo_udf/api/datacube.py

Line 9 in e2fc2c1

from openeo_udf.server.data_model.data_collection_schema import DataCollectionModel

This also creates a dependency on things required by the server, which is undesirable for use cases where someone wants to write a simple UDF. I believe these dependencies can be softened, or removed by moving the required server classes into api.

[huhabla]

The REST based data exchange format must be converted into the internal xarray and pandas based formats. Using the data definitions from the server makes ist easier to implement the conversion. However, the server based format can always be converted into a dictionary that can be used for conversion.
If it is easier for all, then use the dictionary approach, to convert the server side format.