[5.6] Reorder message customization callbacks

[Alexander--]

Call developer-supplied callbacks in Mailer and Mailable a bit earlier, to allow customization of created message body and attachments.

Body and attachments of SwiftMailer messages are opaque encoded blobs, which makes them hard to customize after creation. This PR ensures that callbacks, registered by withSwiftMessage, are executed at earlier point, before SwiftMailer message emits it's body. This enables setting email encoder (8bit/base64 etc.) and other customization of message body.

The change isn't particularly disrupting, because current implementation does not allow to modify the message at all, aside from setting headers. While it's theoretically possible to interact with message from the callback by breaking encapsulation and/or converting full message to string, a brief look at uses of withSwiftMessage in public Github project and Laravel documentation shows that all of them are limited to fiddling with email headers and adding attachments.

Fixes #22982

[domanb]

This change seems to have affected our production code that was previously working great in 5.5. I would send an email, then use the withSwiftMailer callback to get a copy of the message, then use IMAP to copy the sent email to my bosses 'Sent' folder for his email address. Here was the code I was using:

Mail::to($recipient)->send($mail->withSwiftMessage(function($message) use (&$mailString) { $mailString = $message->toString(); }));

After this merge, $mailString just has the headers, no body. How do I go about getting the full email so I can send it off using IMAP?

[Alexander--]

@domanb there seem to be several hooks, that allow to intercept complete message being sent. MessageSending looks particularly promising.

Prior to this change there have been no way to modify a message body before it is built.

[domanb]

Great, thanks Alexander, I'll look into that and see if I can make something work. I put a hacky workaround in place for now, but it's pretty fragile, this looks much more promising. Cheers!