[Improvement] Push notifications routing

[KoenPaas]

Push notifications need to route to the xApp and the corresponding transaction that needs to be signed.

payload: XummPushEventRequest = XummPushEventRequest(
            user_token=xumm_token,
            title=title,
            body=body,
            data={"next": next},
        )
        result = cls.sdk.push.push(payload)
        print(result)

[KoenPaas]

Implemented