* [cli] new 'author template' command
This new command allows users to extract templates for authoring
(customization) without the complexity of finding and downloading a
specific directory for their versioned artifact.
Example usage:
```
openapi-generator author template -g java --library webclient
```
This will write all templates with library-specific templates to the
'./out' directory relative to the current directory.
CLI will refer the user to
https://openapi-generator.tech/docs/templating after generation
* [docs] Usage of author template command
* Log warning if author template fails to output requested library