Change convenience methods to reflect their behavior

[smoyer64]

The RetrieveResourceByExternalId and RetrieveResourceByUserName methods do not "retrieve" resources and return a single instance but rather search for them using a filter and return a ListResponse. The latter of these methods is further complicated by the fact that, of the core resources in the SCIM specification, only the User resource has a userName field.

Perhaps SearchResourcesByExternalId(ResourceType, String) (ListResponse, error) and SearchUsersByUserName(String) (ListResponse, error). Instead of passing the filter as a query parameter, these methods can put them in a SearchRequest and chain to the SearchResource() method.