DSL: Mail namespace & `MailSendingMessageHandler`

DSL:Add SMTP Mail Server

Polishing - PR Comments

DSL: Mail: add `PropertiesConfigurer` lambda

* Move `IntegrationFlowTests` to the `flows` package to avoid cross scanning for components
* Make `MailTests` as integration test and move `Mail.outboundAdapter` to the `IntegrationFlow` `@Bean` definition