AIP-68 Add iframe_views plugin

[bbovenzi]

Simpler than react components for plugins, we allow users to just render a webpage inside of an iframe.

• In our plugin spec, create a new iframe_views type with:
  • name plugin name
  • url url of the page to load in an iframe, allow for template matching for the UI to update like ?run_id={RUN_ID}
  • icon optional file to include for in the nav bar
  • url_route: string, (optional) what to display in the url route, defaults to the plugin name
  • location array of enums to specify where the plugin should be added (only will support full page views "nav", "dag", "dag_run", "task", "task_instance"
• After the plugin type is added, the UI should read the existing GET /plugins api endpoint, determine where to show an iframe plugin and check for any url param matching

[bbovenzi]

Assigning @pierrejeambrun for updated the plugin manager
and myself for the UI