[openthread_border_router] Add ingress support

### Describe the issue you are experiencing

The OpenThread Border Router AddOn has a status webpage, defaults to port `8080` and an api on port `8081` which is useful to check mesh Topology and such.

Please migrate these to the ingress system built into HA.

Right now, the interface must be accessed over `http` because the frontend on port `8080` calls port `8081` over `http` which causes a mixed content error.

Usage of HTTP in browsers is entirely blocked on my domain due to preloaded HSTS.

An additional bug, content is being sent to the user using content-type `text/plain`, which causes `X-Content-Type-Options: nosniff` to block it.

### What type of installation are you running?

Home Assistant OS

### Which operating system are you running on?

Home Assistant Operating System

### Which add-on are you reporting an issue with?

OpenThread Border Router

### What is the version of the add-on?

2.13.0

### Steps to reproduce the issue

1. Try to access `https://ha.example.com/hassio/ingress/core_openthread_border_router`
- fail because not implemented
3. Reverse proxy to the addon using `https://thread-ha.example.com/` to backend url `https://ha.example.com:8080`
- website loads fine but the topology page fails due to trying to load resources via `http://ha.example.com:8081`