[Feature] Enable loading Callbacks from hydra configs

[Xmaster6y]

What does this PR do?

This is a decomposition of #220, which focuses on loading from hydra configs.

Usage:

defaults:
  ...
  - callback@callbacks.c1: my_callback
  - _self_

[Copilot]

Pull request overview

This PR adds infrastructure for loading callback configurations from Hydra config files, establishing a pattern similar to the existing algorithm and model configuration loading mechanisms.

• Introduces CallbackConfig base class for defining callback configurations with YAML loading support
• Adds load_callbacks_from_hydra function to instantiate callbacks from Hydra DictConfig
• Integrates callback loading into the experiment initialization workflow with proper merging of hydra-loaded and programmatically-provided callbacks

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
benchmarl/callbacks/common.py	Defines the CallbackConfig base class with methods for YAML loading and callback instantiation
benchmarl/callbacks/init.py	Initializes the callbacks module with exports and an empty callback_config_registry
benchmarl/hydra_config.py	Adds load_callbacks_from_hydra function and integrates callback loading into load_experiment_from_hydra
benchmarl/init.py	Registers callback schemas with Hydra's ConfigStore for config validation
test/test_callbacks.py	Adds tests for callback loading from Hydra configs, including edge cases for no callbacks and disabled callbacks

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