Refactor `FederatedServer` by introducing `FLRunConfig`.

* Add `declearn.main.config` submodule:
  - Move dataclasses (`TrainingConfig`, `EvaluateConfig`
    and `RegisterConfig`) to this new submodule.
  - Implement `FLRunConfig`, that binds together all arguments and
    hyper-parameters used by the `FederatedServer.run` method, and
    provides with the possibility to write them up as a TOML config
    file (by inheriting `declearn.utils.TomlConfig`).
* Deploy `FLRunConfig` to `FederatedServer`:
  - Replace the argument to `run` and `async_run` by `FLRunConfig`,
    adding the possibility to either provide (legacy-style) kwargs
    or the path to a TOML file to the `run` one.
  - Replace the argument to `initialize` by the full `FLRunConfig`,
    to facilitate overriding by subclasses that may need to send or
    use information on the process in addition to conducting client
    registration.
  - Preserve the use of targetted fields of hyper-parameters in the
    `training_round` and `evaluation_round` methods.