[core] Add API to add messages with attachments

...arproject/briar/android/contact/ConversationActivity.java

...rg/briarproject/briar/api/messaging/MessagingManager.java

...iarproject/briar/api/messaging/PrivateMessageFactory.java

...rg/briarproject/briar/messaging/MessagingManagerImpl.java

...iarproject/briar/messaging/PrivateMessageFactoryImpl.java

...java/org/briarproject/briar/test/TestDataCreatorImpl.java

...arproject/briar/messaging/MessageSizeIntegrationTest.java

...ject/briar/messaging/SimplexMessagingIntegrationTest.java

...oject/briar/headless/messaging/MessagingControllerImpl.kt

...t/briar/headless/messaging/MessagingControllerImplTest.kt