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

script/queue_size.rb

@@ -5,11 +5,11 @@ require_relative "../lib/postal/config"
 require "mysql2"
 
 client = Mysql2::Client.new(
-  host: Postal.config.main_db.host,
-  username: Postal.config.main_db.username,
-  password: Postal.config.main_db.password,
-  port: Postal.config.main_db.port,
-  database: Postal.config.main_db.database
+  host: Postal::Config.main_db.host,
+  username: Postal::Config.main_db.username,
+  password: Postal::Config.main_db.password,
+  port: Postal::Config.main_db.port,
+  database: Postal::Config.main_db.database
 )
 result = client.query("SELECT COUNT(id) as size FROM `queued_messages` WHERE retry_after IS NULL OR " \
                       "retry_after <= ADDTIME(UTC_TIMESTAMP(), '30') AND locked_at IS NULL")