CLI ignore external parser list fix

[kschwab]

Fixes an issue with external parsers that use non-string type list args. They were getting parsed when they should be ignored. Internally, our parser always parses everything into type string, so if pre-parsed args are not of type string they should be ignored.

Simple example of issue:

from argparse import ArgumentParser
from pydantic_settings import BaseSettings, CliSettingsSource

class Cfg(BaseSettings):
    var: str = 'hi'

parser = ArgumentParser()

# External parser using type=int is issue resolved for this fix
parser.add_argument('--num-list', action='append', type=int)

cli_settings = CliSettingsSource(Cfg, root_parser=parser)

parsed_args = parser.parse_args(['--num-list', '1', '--num-list', '2', '--num-list', '3'])
print(parsed_args.num_list)
#> [1, 2, 3]

assert Cfg(_cli_settings_source=cli_settings(parsed_args=parsed_args)).model_dump() == {'var': 'hi'}

Also added support for SimpleNamespace, which is related to but distinct from argparse.Namespace.

[hramezani]

Thanks @kschwab