Only access the connection and form_processor config through their getters

Problem/Motivation

I want to be able to override FormProcessorWebformHandler::getConnection() in order to always return a specific connection on our staging/testing environments, to make sure no production connections are accidentally used.

Proposed resolution

To make this easier, the connection config option should always be accessed through the getConnection() method. And while we're at it, let's do the same for the form_processor option.