Add Author field for Admin-Plugin in Pages Options

[xhs345]

Hi, this is a suggestion for an enhancement.

I noticed that the theme is using a 'author' field in each blog page, which is not included in the Admin Plugin. So the only way is to add or change it manually editing the file (usually by going in Expert mode when using the plugin), which might not be intuitive for beginners

I fixed this for me by adding:

                header.author:
                  type: text
                  label: PLUGIN_ADMIN.AUTHOR

in the file system/blueprints/pages/default.yaml below header.metadata:

Now I'm not sure if this is even possible to change with a theme. And if yes, if it should be in the scope of this theme to change it