Add minimal proxy capabilities to dioxus serve

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.

docs/src/configure.md

@@ -119,6 +119,16 @@ Only include resources at `Dev` mode.
     ]
    ```
 
+### Web.Proxy
+
+Proxy requests matching a path to a backend server.
+
+1. ***backend*** - the URL to the backend server.
+   ```
+   backend = "http://localhost:8000/api/"
+   ```
+   This will cause any requests made to the dev server with prefix /api/ to be redirected to the backend server at http://localhost:8000. The path and query parameters will be passed on as-is (path rewriting is not currently supported).
+
 ## Config example
 
 ```toml
@@ -168,4 +178,7 @@ style = []
 
 # Javascript code file
 script = []
+
+[[web.proxy]]
+backend = "http://localhost:8000/api/"
 ```