FEATURE: Allow oneboxing private GitHub URLs (#27705)

This commit adds the ability to onebox private GitHub
commits, pull requests, issues, blobs, and actions using
a new `github_onebox_access_token` site setting. The token
must be set up in correctly to have access to the repos needed.

To do this successfully with the Oneboxer, we need to skip
redirects on the github.com host, otherwise we get a 404
on the URL before it is translated into a GitHub API URL
and has the appropriate headers added.

spec/lib/onebox/engine/github_blob_onebox_spec.rb

@@ -5,10 +5,12 @@ RSpec.describe Onebox::Engine::GithubBlobOnebox do
     @link =
       "https://github.com/discourse/onebox/blob/master/lib/onebox/engine/github_blob_onebox.rb"
     @uri = URI.parse(@link)
-    stub_request(
-      :get,
-      "https://raw.githubusercontent.com/discourse/onebox/master/lib/onebox/engine/github_blob_onebox.rb",
-    ).to_return(status: 200, body: onebox_response(described_class.onebox_name))
+    @raw_uri =
+      "https://raw.githubusercontent.com/discourse/onebox/master/lib/onebox/engine/github_blob_onebox.rb"
+    stub_request(:get, @raw_uri).to_return(
+      status: 200,
+      body: onebox_response(described_class.onebox_name),
+    )
   end
 
   include_context "with engines"
@@ -38,5 +40,18 @@ RSpec.describe Onebox::Engine::GithubBlobOnebox do
       expect(html).not_to include("/Pages")
       expect(html).to include("This file is binary.")
     end
+
+    context "when github_onebox_access_token is configured" do
+      before { SiteSetting.github_onebox_access_token = "1234" }
+
+      it "sends it as part of the request" do
+        html
+        expect(WebMock).to have_requested(:get, @raw_uri).with(
+          headers: {
+            "Authorization" => "Bearer #{SiteSetting.github_onebox_access_token}",
+          },
+        )
+      end
+    end
   end
 end