Add on_cell_executed hook

[davidbrochart]

• Add a on_cell_executed hook just after a code cell is executed, whether or not it results in an error. Called with kwargs cell, cell_index and execute_reply.
• Add execute_reply to the kwargs passed to the on_cell_error hook.

cc @brichet

[davidbrochart]

The CI failure is unrelated.

cc @devintang3