Fix tests' resilience to missing optional dependencies.
This MR fixes some unit and functional tests to ensure they can run when some optional dependencies are not installed.
- Run
Aggregator
unit tests for available frameworks only. - Skip
gRPC
-dedicated tests when grpc is not installed. - Skip
DPTrainingManager
tests when opacus is not installed. - Skip some
FederatedClient
tests when opacus is not installed. - Fix toy regression functional tests failing when some frameworks are missing due to the use of too-specific type hints.