[TASK] Doctrine: API call to retrieve all connections

Add a new method to the ConnectionPool class to retrieve a list
of all currently configured database connections.

Enhance the system information tooltip to show information about
all database connections instead of only the default connection.

Resolves: #77371
Releases: master
Change-Id: I82948d45c29e8edd47209fde30850f26681404c4
Reviewed-on: https://review.typo3.org/49340


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>