This adds an MVP of some proxying capabilities to the `dioxus serve`
server. The config is similar to that of `trunk serve`: the user can
specify one or more proxy backends under `[[web.proxy]]` in Dioxus.toml,
and the server will intercept requests targeted at the _path_ of that
configured backend and forward them to the backend server.
Example
-------
For example, if the dev server is serving on port 8080 with this config:
```
[[web.proxy]]
backend = "http://localhost:9000/api"
```
then requests to http://localhost:8080/api,
http://localhost:8080/api/ and http://localhost:8080/api/any-subpath
to be forwarded to the respective paths on http://localhost:9000.
This PR doesn't handle path rewriting or anything yet but it would be
fairly simple to add in future if anyone needs it.
