extend-ignore option of the lint subcommand is not configurable via pyproject.toml

The --extend-ignore option is currently only honored when passed via the command line. It is not honored if an extend-ignore option is added to the tool.ni-python-styleguide section of pyproject.toml.

This simple test when added to the test suite will fail:

def test_extend_ignore(styleguide_lint_with_options, tmp_path):
    (tmp_path / "dir1").mkdir()
    (tmp_path / "dir1" / "spam.py").write_text(TOO_LONG_LINE)

    result = styleguide_lint_with_options(lint_args={"extend-ignore": "D100,BLK100"})

    assert result, result.output

Start with the lint test suite and the provided test_extend_ignore case, using the styleguide_lint_with_options fixture. Check how the tool.ni-python-styleguide section of pyproject.toml is read and how extend-ignore from lint_args is handled. Done means the test passes when extend-ignore is configured in pyproject.toml, including D100 and BLK100.