Add sortBy and sortOrder params to additional BrAPI endpoints

[bellerbrock]

Currently sortBy and sortOrder are included in the spec for the studies and trials endpoints, which is useful when importing studies to Field Book. How about adding sort parameters to the variables endpoint for trait imports? It'd be helpful to be able to sort variable responses by observationVariableName and observationVariableDbId. Maybe commonCropName , dataType and status as well.

One limitation to using the sort parameters is knowing if the server has them implemented, and for which attributes. So another request would be to include the details of each individual endpoint's sort implementation within the /serverinfo call, or make it part of a more general compatibility check.