Fix tests' resilience to missing optional dependencies.

- 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.