Fix user cap evaluation for OAuth and LDAP login

What does this MR do and why?

Related to #353534 (closed)

This MR fixes the fact that OAuth sign up should lead to a new user being blocked when the related block_auto_created_users config attribute is set to true, regardless of the sign up user cap.

The key part of the fix here is that we were not reading the correct block_auto_created_users config for the correct protocol. We should have:

• ::Gitlab.config.omniauth.block_auto_created_users which is related to all Omniauth-related connections (see related docs)
• ::Gitlab::Auth::Ldap::Config.new(provider).block_auto_created_users which is related to connections to a specific LDAP directory (see related docs)

This fix is the result of a regression, as mentioned here, when this fix was released.

Screenshots or screen recordings

Scenario 1 - high user cap, "block_auto_created_users: true" and SAML login -> User is blocked.

::Gitlab.config.omniauth.block_auto_created_users returns true.

video_output-2022-02-23_17.16.02.mkv ✅

Scenario 2 - high user cap, "block_auto_created_users: true" and OAuth Gitlab login -> User is blocked.

::Gitlab.config.omniauth.block_auto_created_users returns true.

video_output-2022-02-23_17.14.30.mkv ✅

Scenario 3 - User cap has been reached, "block_auto_created_users: false" and SAML login -> User is blocked.

::Gitlab.config.omniauth.block_auto_created_users returns false.

video_output-2022-02-23_17.44.36.mkv ✅

Scenario 4 - high user cap, "block_auto_created_users: true" for LDAP, "false" for Omniauth. SAML login -> User is active.

::Gitlab::Auth::Ldap::Config.new(provider).block_auto_created_users returns true while ::Gitlab.config.omniauth.block_auto_created_users returns false.

video_output-2022-03-10_15.05.26.mkv ✅

Scenario 5 - high user cap, "block_auto_created_users": "false" for LDAP, "true" for Omniauth. LDAP login -> User is active

This scenario focuses specifically on this issue scenario.

::Gitlab::Auth::Ldap::Config.new(provider).block_auto_created_users returns false while ::Gitlab.config.omniauth.block_auto_created_users returns true.

video_output-2022-02-24_16.03.32.mkv ✅

Scenario 6 - high user cap, "block_auto_created_users": "true" for LDAP, "true" for Omniauth. LDAP login -> User is active

::Gitlab::Auth::Ldap::Config.new(provider).block_auto_created_users and ::Gitlab.config.omniauth.block_auto_created_users both return true

video_output-2022-03-10_15.10.16.mkv ✅

Local setup for testing

Snippet in my local config/gitlab.yml:

development:
  <<: *base
  ldap:
    enabled: true
    servers:
      main:
        label: 'LDAP'
        host: 'gitlab.local'
        port: 10389
        uid: 'uid'
        encryption: 'plain'
        base: 'dc=planetexpress,dc=com'
        block_auto_created_users: true # or false, depending on the scenario.
  omniauth:
    enabled: true
    providers:
    - { name: 'gitlab',
        app_id: '<redacted>',
        app_secret: '<redacted>',
        args: { scope: 'api' } }
    - { name: 'saml',
        label: 'SAML',
        args: {
          assertion_consumer_service_url: 'https://gitlab.local:3443/users/auth/saml/callback',
          idp_cert_fingerprint: '<redacted>',
          idp_sso_target_url: 'http://gitlab.local:8080/simplesaml/saml2/idp/SSOService.php',
          issuer: 'https://gitlab.local:3443',
          name_identifier_format: 'urn:oasis:names:tc:SAML:2.0:nameid-format:persistent'
        }
      }
    allow_single_sign_on: ['gitlab', 'saml']
    auto_link_saml_user: true
    block_auto_created_users: true # or false depending on the scenario

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #338980 (closed)