Add a tuple read format for Map columns

[Kayvan-Zahiri]

Closes #949.

A ClickHouse Map is not a unique-key collection, so materializing every one as
a Python dict drops pairs. This adds an opt-in tuple read format that returns
the pairs as a list of tuples.

I checked this against the server source rather than inferring it from a JSON
round trip. At v26.3.21.7-lts:

• src/Columns/ColumnMap.h describes ColumnMap as a column that stores a
  nested Array(Tuple(key, value)) column
• src/DataTypes/DataTypeMap.cpp builds that nested type as
  DataTypeArray(DataTypeTuple({key_type, value_type}, {"keys", "values"}))
• neither file deduplicates keys anywhere

So a list of (key, value) tuples is the server's own representation, not a
convention I picked.

Behavior:

read format	map('k','1','k','2')
native (default, unchanged)	{'k': '2'}
tuple	[('k', '1'), ('k', '2')]

Enable per query with query_formats={'Map': 'tuple'} or globally with
set_read_format('Map', 'tuple').

Tests are parametrized over the shapes the issue names: top level Map,
Array(Map(...)), and a Map nested as another map's value. Each shape is
asserted in both formats, so the native cases pin that the default did not
move. Four fail without the change. pytest tests/unit_tests goes from 1453 to
1460 passing with the 37 pre-existing failures unchanged, ruff and mypy are
clean on the changed files.

Not included: insert side handling of duplicate keys, and sync/async integration
coverage against a live server, which I could not run here. Happy to add the
integration tests if you want them in this PR.