FEATURE: Allow users to sign in using LinkedIn OpenID Connect (#26281)

LinkedIn has grandfathered its old OAuth2 provider. This can only be used by existing apps. New apps have to use the new OIDC provider.

This PR adds a linkedin_oidc provider to core. This will exist alongside the discourse-linkedin-auth plugin, which will be kept for those still using the deprecated provider.

spec/lib/auth/linkedin_oidc_authenticator_spec.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+RSpec.describe Auth::LinkedInOidcAuthenticator do
+  let(:hash) do
+    OmniAuth::AuthHash.new(
+      provider: "linkedin_oidc",
+      extra: {
+        raw_info: {
+          email: "100",
+          email_verified: true,
+          given_name: "Coding",
+          family_name: "Horror",
+          picture:
+            "https://media.licdn.com/dms/image/C5603AQH7UYSA0m_DNw/profile-displayphoto-shrink_100_100/0/1516350954443?e=1718841600&v=beta&t=1DdwKTzW2QdVuPtnk1C20oaYSkqeEa4ffuI6_NlXbB",
+          locale: {
+            country: "US",
+            language: "en",
+          },
+        },
+      },
+      info: {
+        email: "coding@horror.com",
+        first_name: "Coding",
+        last_name: "Horror",
+        image:
+          "https://media.licdn.com/dms/image/C5603AQH7UYSA0m_DNw/profile-displayphoto-shrink_100_100/0/1516350954443?e=1718841600&v=beta&t=1DdwKTzW2QdVuPtnk1C20oaYSkqeEa4ffuI6_NlXbB",
+      },
+      uid: "100",
+    )
+  end
+
+  let(:authenticator) { described_class.new }
+
+  describe "after_authenticate" do
+    it "works normally" do
+      result = authenticator.after_authenticate(hash)
+      expect(result.user).to eq(nil)
+      expect(result.failed).to eq(false)
+      expect(result.name).to eq("Coding Horror")
+      expect(result.email).to eq("coding@horror.com")
+    end
+  end
+end