Add fuzzer for datetime module

[AdamKorcz]

Adds a structured fuzzer for the _datetime C module (Modules/_datetimemodule.c). It has two main branches: parsing and formatting. The parsing branch feeds fuzzed Unicode strings into date.fromisoformat(), time.fromisoformat(), and datetime.fromisoformat() to stress the C-level ISO 8601 parser. The formatting branch constructs valid date, time, and datetime objects with fuzzed field values and then calls strftime() with fuzz-generated format strings, exercising the C formatter's handling of arbitrary directives and edge-case field combinations.