[FEATURE] Add Symfony mailer DSN configuration option to install tool

The install tool has now the possibility to configure
the Symfony mailer with DSN.

The DSN setting was already implemented but with a not
consistent configuration option and was never documented.

Resolves: #93668
Releases: master
Change-Id: I12428aad0c41af74471f25efe3462632866c29a7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68197


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>