Hardcoded MeanAggregator for the federated server #1238

Description:

The MeanAggregator is hardcoded in the federated controller. It means that even if we pass the "byzantine" strategy in the description of the task, the mean strategy will be used. Also, this does not allows us to specify the parameters of the mean aggregator: roundCutoff, threshold, thresholdType currently at 0, 1 and "relative" respectively.

Fix:

Build the aggregator from the task fields. Similar work has been made for the clients: see getAggregator() in discojs/src/aggregator/get.ts. We may be able to utilize this function directly. However, the default value don't seem to match the ones for a federated server...