add a method to check consistency between secret channel keys in SDK and public channel keys on server

In packages/crypto/src/classes.

If there is a secret key not having a matching public key it probably means the call to postChannels in method createNewChannels of CryptoContext failed for some reason, but it is not too serious because we can re-generate the public key from the secret one and upload it again to the server.

If however there is a channel public key on server for which we don't have the secret key I don't know what happened but this would be a much more serious situation!

Not sure when we would call this method, though, we have to think about this.