Make `doctrine/dbal` an optional or suggested package

[johanib]

Currently, when using this bundle, it adds a composer dependency, doctrine/dbal. However, some projects, like stepup-azuremfa, do not use a database.
But the monitor bundle causes doctrine/dbal to be installed in those projects.

This is not really a problem, as the connection is optional in the DoctrineConnectionHealthCheck.

        if ($this->connection === null) {

But it would be nice to not packages without reason.

Possible solution:

1. Move doctrine/dbal from require to suggest in composer.json.
2. Exclude DoctrineConnectionHealthCheck.php from resource discovery in services.yaml to prevent autoloading errors?
3. Conditionally register DoctrineConnectionHealthCheck in OpenConextMonitorBundle.php only if Doctrine\DBAL\Connection exists.