[BUG] Failure to parse --auth command line option

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Unable to properly pass --auth header from the command line.

openapi-generator version

Running on macOS 10.15.5

% openapi-generator version
4.3.1

Command line used for generation

% openapi-generator generate --input-spec https://mydomain/rest/swagger.json --generator-name python --output out --auth "Authorization: Basic dXNlcjp1c2Vy"
[error] Found unexpected parameters: [Basic, dXNlcjp1c2Vy]

See 'openapi-generator-cli help' for usage.

Expected

I would expect the --auth parameter to be parsed correctly. Also, there don't seem to be any documentation examples on using this parameter to compare to.

Start by reproducing the openapi-generator generate command with the shown --auth value and inspect the command-line parsing entry point. Confirm that the header value is accepted as one argument, then verify the same command completes without the unexpected-parameters error and add or update a documentation example if the relevant CLI documentation is identified.