This commit changes the API of the SampleServer's mock
registry s.t. one can register a mocked version of a method,
whatever the model. The former API required a specific
model to be specified, and only applied on this model.

However, in order to enable the sample data feature on the
cohort view, the 'get_cohort_data' method needs to be
mocked, whatever the concerned model.

One can now register mocked methods for a specific
model only (with key 'model/method') or for every
models (with key 'method').

Task ID: 2282196