svc: Add navigate operation

The *POST /screen/{name}/navigate* path operation allows the client to
request the screen navigate to a different URL, specified by the `url`
form field.

svc/src/hudctrl/api.py

@@ -126,6 +126,14 @@ async def get_screenshot(
     return response
 
 
+@app.post(
+    '/screen/{name}/navigate',
+    response_class=fastapi.responses.PlainTextResponse,
+)
+async def navigate(name: str, url: str = fastapi.Form(...)):
+    await svc.navigate(name, url)
+
+
 @app.on_event('shutdown')
 async def on_shutdown():
     await svc.shutdown()

svc/src/hudctrl/hud.py

@@ -116,6 +116,12 @@ class HUDService:
                     dict,
                 )
 
+    async def navigate(self, name: str, url: str) -> None:
+        assert self.marionette
+        async with self.lock:
+            await self.marionette.switch_to_window(self.windows[name])
+            await self.marionette.navigate(url)
+
     async def refresh_screen(self, name: str) -> None:
         assert self.marionette
         async with self.lock: