Add ability to attach SSH key to plan's subversion repository configuration

Currently a user can only specify the location of an SSH key local to an agent for Subversion. We don't want to have to store SSH keys on agents (as this is not secure, and is not a good model when there are many accounts and SSH keys), and would like to be able to attach an SSH key to a plan similar to the way Git repositories are configured with SSH. Please provide the facility to attach an SSH key to a plan for Subversion.