Handle SAS verification by adding an endpoint where the decimal SAS code generated by another device can be sent. If this code matches with neb's generated code, the verification succeeds and the device is marked as trusted.
Add a cryptotest service which can be used for other clients to test their functionalities related to e2ee. This currently includes a challenge / response, key forwarding functions (key request / key forward) and SAS verification. SSSS and cross-signing are planned for the future.