Allow omniauth username claim to be configurable

What does this MR do and why?

This MR allows administrators to specify a custom claim from a provider's auth_hash to use as the username in GitLab.

This allows self-hosted instances to handle auth_hash mappings outside of their control (i.e. from external identity providers or omniauth providers). For example, GitLab's OpenID Connect provider exposes a subset of data in info and remaps others. If the desired username is returned in a field outside of that set, like in sub, there's no way to retrieve the correct username.

This is not to be confused with the configurable uid_field mentioned in the docs:

<uid_field> (optional) is the field name from the user_info.raw_attributes that defines the value for uid. For example, preferred_username. If this value is not provided or the field with the configured value is missing from the user_info.raw_attributes details, the uid uses the sub field.

This option affects the user's extern_uid.

Screenshots or screen recordings

Here's a sample where I changed my username to my gitlab.com uid:

How to set up and validate locally

With gdk and this MR checked out:

1. Create an application on gitlab.com to use as an authentication provider (using read_user scope for minimal access)
2. Modify the config/gitlab.yml for gitlab in your gdk setup with the application credentials from step 1 and the following args:

development:                                                                    
  <<: *base                                                                         
  omniauth:                                                                         
    block_auto_created_users: false                                                 
    allow_single_sign_on: ['gitlab']                                            
    providers:                                                                  
    - { name: 'gitlab',                                                         
        app_id: 'APP_ID',
        app_secret: 'APP_SECRET',
        args: { gitlab_username_claim: 'uid', scope: 'read_user' } } # using uid as it is available and not generally used

3. Run gdk open and click sign-in with GitLab button
4. After signing in, click the user avatar in the top-right corner, the username should now be your uid (instead of the same name on gitlab.com, see screenshot).

MR acceptance checklist

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