Configurable push gateway

### Your use case

I recently started self-hosting a Matrix homeserver and thought I had completely dropped dependency on "matrix.org" infrastructure, but then I realised that all Element apps are [hard-configured](https://github.com/vector-im/element-android/blob/b9934d7239fcd46dcb3989e2a948e4ecce79d174/vector-config/src/main/res/values/config.xml#L20) to use the "matrix.org" push gateway.

Are there any plans to make self-hosting a push gateway (e.g. Sygnal) more feasible for non-app developers?

I've come across the [Worth noting](https://github.com/matrix-org/sygnal/blob/2a2b3ebc61ade813971cd454acdb85fc6d1141c9/docs/applications.md#worth-noting-1) section of Sygnal docs, but I'm hoping there is a way for power users to be able to self-host a push gateway and not have to build their own clones of the Element apps.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_