Add observation_function for full custom control over the observation

Lets a game define how its state becomes an observation as a plain
Python function (module:attr), used identically by GameEnvironment
(training) and TrainedPolicy (inference) instead of two independently
maintained encoding paths. Removes the egocentric/egocentric_player/
egocentric_radius flags — cropping is now something an
observation_function does itself by calling egocentric_board(), and
extras_size is discovered from one sampled observation instead of
being configured via observe_state_sizes.

docs/api.rst

@@ -7,7 +7,7 @@ Game description
 ----------------
 
 .. autoclass:: retro_gamer.GameMetadata
-   :members: from_pyproject, from_dict, validate
+   :members: from_pyproject, from_dict, validate, resolve_observation_function
 
 Training
 --------