Add Operations and BatchOperations stub methods

Add Operations and BatchOperations stub methods

Updated stub generator script to also add stubs method definitions
for the :class:`.Operations` class and the :class:`.BatchOperations`
class obtained from :meth:`.Operations.batch_alter_table`.

Repaired the return signatures for :class:`.Operations` that mostly
return ``None``, and were erroneously referring to ``Optional[Table]``
in many cases.

Fixes: #1093
Change-Id: I98d38dd5a1e719b4dbbc1003746ec28f26c27808