Add a store_time flag to get and tag filtering to iterkeys

[avgvi]

Two additive features requested upstream.

• store_time on get (upstream issue 330): Cache.get, FanoutCache.get and DjangoCache.get take store_time=False as their last parameter. When true, the store time is appended after the value and any other requested metadata, in the order expire time, tag, store time, so every existing tuple shape is unchanged when the flag is off; misses and FanoutCache shard timeouts return None for each requested field. One helper builds the tuple for hits, misses and timeouts; the get query selects one more column. pop has its own flags and is untouched.
• iterkeys(tag=...) (upstream issue 341): upstream pull request 372 merged with its original commit. tag defaults to the ENOVAL sentinel meaning no filter; tag=None selects untagged rows; the filter runs inside the existing key pagination, keeps reverse semantics and fetches no values. It brought parametrized tests and a tutorial section with doctests. FanoutCache has no iterkeys, so nothing to pass through.

Tests: all tuple shapes on the three classes including misses and the FanoutCache timeout path, a file-like value, and the Django variant; the tutorial gains a store_time paragraph with a doctest.

Verified locally: 384 tests pass at 98.85 % on Python 3.12 with Django 4.2 and on 3.14 with Django 6.0, golden fixtures included; bluecheck, isortcheck, flake8, mypy, pylint, doc8, rstcheck and the Sphinx build pass.