Accept date strings in test data for assurance tests

[milanwiedemann]

Currently dates have to be specified as datetime.date objects but we would like to also accept date strings so that dates in assurance tests can be specified like this:

    1: {
        "patients": [{"date_of_birth": "1950-01-01"}],
        "expected_in_population": True,
        "expected_columns": {
            "age": 73,
        },
    },

This currently fails with an AttributeError when using the test patient above to calculate age using .age_on in the dataset definition:

Traceback (most recent call last):
  File "/opt/venv/bin/ehrql", line 8, in <module>
    sys.exit(entrypoint())
             ^^^^^^^^^^^^
  File "/app/ehrql/__main__.py", line 67, in entrypoint
    return main(sys.argv[1:], environ=os.environ)  # pragma: no cover
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/__main__.py", line 92, in main
    function(**kwargs)
  File "/app/ehrql/main.py", line 326, in assure
    results = assurance.validate(variable_definitions, test_data)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/assurance.py", line 63, in validate
    query_results = {
                    ^
  File "/app/ehrql/assurance.py", line 63, in <dictcomp>
    query_results = {
                    ^
  File "/app/ehrql/query_engines/in_memory.py", line 32, in get_results
    table = self.get_results_as_table(variable_definitions)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory.py", line 56, in get_results_as_table
    col = self.visit(node)
          ^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory.py", line 86, in visit
    value = visitor(node)
            ^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory.py", line 313, in visit_DateDifferenceInYears
    return self.visit_binary_op_with_null(node, date_utils.date_difference_in_years)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory.py", line 203, in visit_binary_op_with_null
    return self.visit_binary_op(node, handle_null(op))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory.py", line 192, in visit_binary_op
    return apply_function(op, lhs, rhs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory_database.py", line 457, in apply_function
    {p: fn(*[col[p] for col in columns]) for p in patients},
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory_database.py", line 457, in <dictcomp>
    {p: fn(*[col[p] for col in columns]) for p in patients},
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ehrql/query_engines/in_memory_database.py", line 503, in fn_with_null
    return fn(*values)
           ^^^^^^^^^^^
  File "/app/ehrql/utils/date_utils.py", line 30, in date_difference_in_years
    year_diff = end.year - start.year
                           ^^^^^^^^^^
AttributeError: 'str' object has no attribute 'year'