Implement Mutiny support for the mailer service

[cescoffier]

• Implement the Mutiny API
• Deprecated the previous API
• Update tests and documentation

These changes are backward compatible.

I've also updated:

• the mock mailer to print <empty> instead of null
• fix a few logged messages

The template support still returns a CompletionStage. It will be covered once #7101 is merged. This would be a breaking change.

[gastaldi]

Perhaps the returned object may give more info instead of Void? MailResult perhaps?

[cescoffier]

And what would it contains? Basically it just tells you success or failure which is conveyed using the Uni events.

[gastaldi]

I was thinking about the message itself but maybe Uni already handles that

[cescoffier]

Force pushed to fix the import ordering issue.

[gsmet]

I just had one small Javadoc comment. Otherwise looks good.

Feel free to fix and merge or ignore and merge :).

[gsmet]

I fixed the small typo and rebased. Let's wait for CI.

[Akejelu-Nelu]

I have difficulty in getting the response code for each email i sent with reactive mailer.

return reactiveMailer.send(mail)
.subscribeAsCompletionStage()
.thenApply(response -> Response.accepted().build())
.exceptionally(throwable -> Response.serverError().build());

this is what my return statement looks like after building the mail. The mail object also has a list of recipients. I'm not sure how i can get the response code / status of that email if it was sent or not. Any suggestion is appreciated.