Server configuration enhancements

[mryab]

Right now, it's tricky to start a Server with a custom expert and to change optimizer/scheduler parameters easily without modifying the code.

• Implement hierarchical YAML configuration that will allow to specify parameters for expert layers/optimizers/schedulers
• Switch to something like Hydra for simplicity?
• Get rid of default config, add an example to configuration
• Ideally we don't want to specify all parameters in Server.create() and in hivemind_cli.run_server twice