Allow setting env_nested_delimiter for CliSettingsSource

The delimiter is hardcoded to '.' for CliSettingsSource, but in some cases it would be helpful to allow the user to adjust that default!

https://github.com/pydantic/pydantic-settings/blob/9380bc653d0d5b712ea85230608854d898947ec8/pydantic_settings/sources/providers/cli.py#L234C13-L234C33