PrivilegedAggregationFunction Interface

[psfoley]

This PR:

• Creates a new PrivilegedAggregationFunction interface.
  • Provides direct access to the aggregator's dataframe via the tensor_db parameter (instead of db_iterator)
  • Adds three convenience functions to the passed tensor_db dataframe: store, retrieve, and search
    • These remove significant boilerplate for each aggregation function
  • Is located in the new openfl.component.aggregation_functions.experimental package to reflect it's experimental status that is subject to change
• Adds necessary documentation for the interface, and amends the Custom Aggregation Tutorial to include a working example
• Is implemented to support innovative algorithms in the FeTS Challenge