Introduce an "generate-config" command that dumps a default config file

Introduce a "generate-config" command that dumps a default config file.

This should emit all the configuration options.

Required ones should have a reasonable value.
Optional ones should be commented out, and have their default value there.

With this, the user can more easily uncomment/customize values before digging through docs and the config file syntax.

Start by locating the command entry point and the existing configuration definition and syntax used by swift-openapi-generator. Ensure the new generate-config command emits every option, with reasonable values for required options and commented defaults for optional ones; verify the generated file can be customized using the documented config syntax.

Written by the indexing model from the issue text.