Add TreeSpec to support dataclass-based nested specs (Issue #13)

[Pranav-0440]

This PR adds a minimal TreeSpec implementation to dm_env to support structured action and observation specifications using Python dataclasses. This directly addresses Issue #13 ("Tree Spec").

✔ What is added

• A new TreeSpec class in specs.py.
• A recursive _generate_tree_value() helper to generate valid test values.
• Support for nested structures:
  • dataclasses
  • dicts
  • tuples
  • lists
  • existing dm_env specs (Array, BoundedArray, DiscreteArray, etc.)
• A new test: test_tree_spec_generate_value.

✔ Why this is useful

Many JAX-based RL environments define structured I/O using dataclasses. While dm_env already supports tuple/dict specs, dataclass-based specs were not recognized, causing type checking and clarity issues. TreeSpec provides an explicit and type-safe way to represent these structures.

✔ Compatibility

This feature is fully backward-compatible and does not modify any existing spec behavior. All existing tests pass (166 passed).