Add dynld proxy

[James-Mart]

Simplifies when UIs that are meant to be the target of a reverse proxy serve plugins.

Current workaround is:
https://github.com/gofractally/psibase/blob/247f7898243a0933a9c2def8a98bfb179bda6687/packages/user/Fractals/service/src/helpers/link_fractal_core_plugin_deps.rs

If possible would be good to not couple the service to plugin deps