Add an interface for Drupal\Core\Database\Connection

Problem/Motivation

I have been wondering for a long time why there's no interface for the Drupal\Core\Database\Connection class. When injecting dependencies, it's always one of the only classes that doesn't have an interface. Is this an intentional decision or something that was forgotten? I came across a comment from @dww → about the same thing in 2009, but apart from that I haven't found any discussions in the issue queue.

Proposed resolution

Add Drupal\Core\Database\ConnectionInterface.