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.

uv.lock

@@ -1154,7 +1154,7 @@ wheels = [
 
 [[package]]
 name = "retro-gamer"
-version = "0.1.1"
+version = "0.2.0"
 source = { editable = "." }
 dependencies = [
     { name = "click" },