Allow multiple plugin names

Allow multiple plugin names

The :class:`.URL` object now allows query keys to be specified multiple
times where their values will be joined into a list.  This is to support
the plugins feature documented at :class:`.CreateEnginePlugin` which
documents that "plugin" can be passed multiple times. Additionally, the
plugin names can be passed to :func:`.create_engine` outside of the URL
using the new :paramref:`.create_engine.plugins` parameter.

Change-Id: Ifc48ad120bd6c6204eda567492caf79832aeeaa5
Fixes: #4170