Create and lock user role in Module

Question

I am building my first D8 module, so I probably need a little kick off. My Module should do the following things:

1. Create a user role
2. Extend the the profile form for users, who have this particular role.

To start I created /modules/extuser/extuser.info.yml with this content:

   langcode: en
   status: true
   dependencies: {  }
   id: ext_member
   label: ext-member
   is_admin: null
   permissions: {  }

After the module was installed the role appeared in the list, just fine! But what really would like to have, is that this role is locked, so that it cannot be deleted using the UI, just as the "anonymous" is.

First question: How can I lock that role?

The second thing will be: Adding fields to that role. So right here my knowledge about the new internals ends and really would like to know if that can be done with configuration only, or if I have to code. Any hint to show how it could be done would be very fine.