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
DSL: Mail Inbound Adapters

Add POP3 Server for Tests

Polishing; PR Comments; IMAP4 Server
DSL: Mail Polish - Add SearchTerm (lambda) To Test
DSL: Mail - Add IMAP Idle Adapter

Polish IMAP test server and update it to support IDLE.
Polishing