Use matrix.org's push gateway by default with UnifiedPush

### Your use case

#### What would you like to do?

Sygnal, the push gateway behind `https://matrix.org/_matrix/push/v1/notify` supports WebPush. This can be used to push to UnifiedPush endpoints, which is compatible with WebPush

#### Why would you like to do it?

This avoid relying on an external service (the current push gateway hosted by unifiedpush.org)

_This gives E2EE to the notifications_ which is a good point regarding security

#### How would you like to achieve it?

- Add a new appId to matrix.org's [sygnal config](https://github.com/matrix-org/sygnal/blob/main/sygnal.yaml.sample):
- (example) name: im.vector.app.unifiedpush
- type: webpush
- The pusher data have to be updated see (https://github.com/element-hq/hydrogen-web/blob/9b68f30aad329c003ead70ff43f289e293efb8e0/src/platform/web/dom/NotificationService.js#L32).
- Use UnifiedPush's new release of the library ([3.0.0-rc2](https://central.sonatype.com/artifact/org.unifiedpush.android/connector/3.0.0-rc2)) which does the notifications decryption
- Matrix.org's sygnal may have to be configured to accept all endpoints (which is the goal of webpush). It is not a security issue (SSRF) if the server can't access any internal networks or local services that aren't exposed on the Internet. Else, local IP must be blocked, [like synapse does](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#ip_range_whitelist).
- Change the [default gateway](https://github.com/element-hq/element-x-android/blob/61c021668619fa400784c3ec5c8098baf1089c90/libraries/pushproviders/unifiedpush/src/main/kotlin/io/element/android/libraries/pushproviders/unifiedpush/UnifiedPushConfig.kt#L24) to use `https://matrix.org/_matrix/push/v1/notify`

And it should work.

#### Note

The discovery request to catch self-hosted gateway is still used and available

### Have you considered any alternatives?

A new push kind would be added to matrix specifications to allow webpush, that would bring encrypted push messages in the same time (https://github.com/matrix-org/matrix-spec-proposals/pull/3013)

### Additional context

-

### Are you willing to provide a PR?

No