Add type annotations to indexable extension code

Add type annotations to indexable extension code

A typing test case (plain_files/ext/indexable.py) is also added.

In order to make the methods of index_property conform with type
definitions of `fget`, `fset` and `fdel` arguments of hybrid_property,
we need to make the signature of protocols
(e.g. `_HybridGetterType`) `__call__`) method positional only.

Related to #6810.

Closes: #12763
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12763
Pull-request-sha: 896b9642629fb212a71e68ed3f08705d08407dbf

Change-Id: I42980ccf20ff9c992ebd616f38545e06ede57d36