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.

src/error.rs

@@ -32,6 +32,12 @@ pub enum Error {
     #[error("{0}")]
     CustomError(String),
 
+    #[error("Invalid proxy URL: {0}")]
+    InvalidProxy(#[from] hyper::http::uri::InvalidUri),
+
+    #[error("Error proxying request: {0}")]
+    ProxyRequestError(hyper::Error),
+
     #[error(transparent)]
     Other(#[from] anyhow::Error),
 }