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.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "retro-gamer"
-version = "0.1.1"
+version = "0.2.0"
 description = "A toolkit for learning reinforcement learning by training agents to play retro games"
 readme = "README.md"
 requires-python = ">=3.11"