Create iframe UI plugins

[bbovenzi]

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

This will take the same spec as react components as specified in #42702 but the ui_view.type needs to be "iframe" and an iframe_link needs to be specified.

Then a GET /plugins endpoint on FastAPI will need to read from the plugins manager and send the plugin spec to the UI. The UI can then render the url in an iframe.

Later on, we will need to add support passing props to the iframe via url parameters to load the right page.