Add docs for debug mitm proxying. (#1490)

docs/debug_proxying.md

@@ -0,0 +1,22 @@
+# Setup a debug mitm proxy to inspect all the app's network traffic
+
+1) Install mitmproxy: `brew install mitmproxy`.
+    1) Launch `mitmweb` from a terminal. It will pop up mitmproxy's web interface in a web browser.
+1) Configure Android Emulator.
+    1) Launch your android emulator.
+    1) Open its settings page and go to Settings -> Proxy (nb this tab isn't visible when running the emu inside the Android Studio window, you need to set it so it runs in its own window).
+    1) Disable "Use Android Studio HTTP proxy settings" and pick "Manual proxy configuration".
+    1) Set `127.0.0.1` as "Host name" and `8080` as "Port number".
+    1) Click "Apply" and verify that "Proxy status" is "Success" and close the settings window.
+       <img width="932" alt="Screenshot 2023-10-04 at 14 48 47" src="https://github.com/vector-im/element-x-android/assets/1273124/bf99a053-20b0-42a4-91d3-9602f709f684">
+1) Install the mitmproxy CA cert (this is needed to see traffic from java/kotlin code, it's not needed for traffic coming from native code e.g. the matrix-rust-sdk).
+    1) Open the emulator Chrome browser app
+    1) Go to the url `mitm.it`
+    1) Follow the instructions to install the CA cert on Android devices.
+       <img width="606" alt="Screenshot 2023-10-04 at 14 51 27" src="https://github.com/vector-im/element-x-android/assets/1273124/5f2b6f27-6958-4ea7-97fe-c7f06d105da5">
+1) Slightly modify the Element X app source code.
+    1) Go to the `RustMatrixClientFactory.create()` method.
+    1) Add `.disableSslVerification()` in the `ClientBuilder` method chain.
+1) Build and run the Element X app. 
+1) Enjoy, you will see all the traffic in mitmproxy's web interface.
+   <img width="1110" alt="Screenshot 2023-10-04 at 14 50 03" src="https://github.com/vector-im/element-x-android/assets/1273124/5d039efd-448d-426c-a384-dbbceb9f33ac">