[FEATURE] Add signal for mailer initialization

This signal allows for additional processing upon initialization
of a mailer object, e.g. registering a Swift mailer plugin.

Resolves: #62960
Releases: master
Change-Id: I048de8ef422911d5863ab2a05312dc7f8c7d195f
Reviewed-on: http://review.typo3.org/34176
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>