Add support for using dynamic extensions in static plugins

Console static plugins, maintained as part of the frontend monorepo, currently use static extension types (packages/console-plugin-sdk/src/typings) which directly reference various kinds of objects, including React components and arbitrary functions.

To ease the long-term transition from static to dynamic plugins, we should support a use case where an existing static plugin goes through the following stages:

1. use static extensions only (we are here)
2. use both static and dynamic extensions
3. use dynamic extensions only (ideal target for 4.7)

Once a static plugin reaches the "use dynamic extensions only" stage, its maintainers can move it out of the Console monorepo - the plugin becomes dynamic, shipped via the corresponding operator and loaded by Console app at runtime.