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/background.rst

@@ -351,6 +351,13 @@ engineering decisions live: what derived quantities should the agent
 see, and does giving it those values give it an advantage a human
 player would not have?
 
+``character_set``/``observe_state`` cover the common cases, but
+sometimes you want full control over how the board becomes numbers — for
+example, cropping it to a window centered on the agent rather than always
+seeing the whole thing. ``observation_function`` (see :doc:`reference`) lets
+you write that transformation as ordinary code instead of a combination of
+flags.
+
 Neural network architectures
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~