Fix RemovedInMarshmallow4Warning by moving schema fields description to metadata

[FedericoNegri]

To fix the many RemovedInMarshmallow4Warning we should move the marshmallow field description to the metadata, e.g.

num_trials = fields.Int(
    allow_none=True, 
    description="Maximum number of attempts to execute the task."
)

should become

num_trials = fields.Int(
   allow_none=True, 
   metadata={"description":"Maximum number of attempts to execute the task."}
)