Add LTI 1.3 Fields to support Tools that can provide ToolSet URLs

While it is not required for LTI 1.3, I am going to add a feature to the Sakai LTI 1.3 data model to support KeySet URLs from the tool in the future.   KeySet urls allow a tool to rotate their public / private keys and inform the LMS of their new public key at any time.  LTI 1.3 requires the LMS needs to provide the tool a KeySet url and a future version of LTI will likely insist that the LMS allow a tool to provide a KeySet URL so I am going to get in front of that and add the data model now so I can add the feature in a minor Sakai-19 release if it becomes necessary.  I may or may not get to the actual UI and run-time for tool-provided KeySet urls by Sakai-19.  Since it is not required by LTI 1.3 or LTI Advantage - it has lower priority than other LTI 1.3 features.