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/config.rs

@@ -63,6 +63,7 @@ impl Default for DioxusConfig {
                     title: Some("dioxus | ⛺".into()),
                     base_path: None,
                 },
+                proxy: Some(vec![]),
                 watcher: WebWatcherConfig {
                     watch_path: Some(vec![PathBuf::from("src")]),
                     reload_html: Some(false),
@@ -97,6 +98,7 @@ pub struct ApplicationConfig {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct WebConfig {
     pub app: WebAppConfig,
+    pub proxy: Option<Vec<WebProxyConfig>>,
     pub watcher: WebWatcherConfig,
     pub resource: WebResourceConfig,
 }
@@ -107,6 +109,11 @@ pub struct WebAppConfig {
     pub base_path: Option<String>,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct WebProxyConfig {
+    pub backend: String,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct WebWatcherConfig {
     pub watch_path: Option<Vec<PathBuf>>,