DSL: Add `enrichHeaders(Map)` EIP-method

* Introduce `MapBuilder` and its custom inheritor - `MailHeadersBuilder`
* Add `HeaderEnricherSpec.headers(MapBuilder)` and `.headers(Map)`
* Change `MailTests` to use new `Mail.headers()` and new `IntegrationFlowDefinition.enrichHeaders(MapBuilder)`
* Change `mailapi` dependency to the IO's `javax.mail`