Fix cookie samesite issue for LTI 1.3 deep linking

LTI 1.3 deep linking don't seem to be working in latest Chrome. Seemingly this is due to because SameSite property of session and CSRF token cookies are not set to None to allow cross-site. Even though the deep link selection iframe ends up at same domain (Canvas) it's navigated to through the tool domain which effectively makes it cross-site.

Closes instructuregh-1900

Test plan:
- Test LTI 1.3 deep linking with an external tool
- Test compatibility with different browsers (new browsers supporting SameSite: None and browsers not supporting)
- Investigate any undesired security implications this change might have

gems/canvas_breach_mitigation/lib/canvas_breach_mitigation/masking_secrets.rb

@@ -32,7 +32,7 @@ def masked_authenticity_token(cookies, options = {})
         encoded_masked_token = masked_token(unmasked_token(cookies["_csrf_token"]))
 
         cookie = { value: encoded_masked_token }
-        %i[domain httponly secure].each do |key|
+        %i[domain httponly secure same_site].each do |key|
           next unless options.key?(key)
 
           cookie[key] = options[key]

gems/request_context/lib/request_context/session.rb

@@ -32,7 +32,8 @@ def call(env)
         ActionDispatch::Request.new(env).cookie_jar[:log_session_id] = {
           value: session_id,
           secure: Rails.application.config.session_options[:secure],
-          httponly: true
+          httponly: true,
+          same_site: Rails.application.config.session_options[:same_site]
         }
       end
 

lib/canvas/request_forgery_protection.rb

@@ -40,7 +40,7 @@ def verified_request?
 
     def authenticity_token_options
       session_options = CanvasRails::Application.config.session_options
-      options = session_options.slice(:domain, :secure)
+      options = session_options.slice(:domain, :secure, :same_site)
       options[:httponly] = HostUrl.is_file_host?(request.host_with_port)
       options
     end