feat: new configuration system (and schema) (#2819)

spec/lib/smtp_server/client/data_spec.rb

@@ -39,7 +39,7 @@ module SMTPServer
         client.handle("RCPT TO: #{route.name}@#{route.domain.name}")
         Timecop.freeze do
           client.handle("DATA")
-          expect(client.headers["received"]).to include "from test.example.com (1.2.3.4 [1.2.3.4]) by postal.example.com with SMTP; #{Time.now.utc.rfc2822}"
+          expect(client.headers["received"]).to include "from test.example.com (1.2.3.4 [1.2.3.4]) by #{Postal::Config.postal.smtp_hostname} with SMTP; #{Time.now.utc.rfc2822}"
         end
       end
 
@@ -72,7 +72,7 @@ module SMTPServer
             client.handle("This is some content for the message.")
             client.handle("It will keep going.")
             expect(client.instance_variable_get("@data")).to eq <<~DATA
-              Received: from test.example.com (1.2.3.4 [1.2.3.4]) by #{Postal.config.dns.smtp_server_hostname} with SMTP; #{Time.now.utc.rfc2822}\r
+              Received: from test.example.com (1.2.3.4 [1.2.3.4]) by #{Postal::Config.postal.smtp_hostname} with SMTP; #{Time.now.utc.rfc2822}\r
               Subject: Test\r
               \r
               This is some content for the message.\r