Deprecating the publicKeyBase64 field in the authentication protocol (since 0.4)

Before ContainerSSH version 0.4 sent a field called publicKeyBase64 to the authentication server which contained the SSH key in the binary OpenSSH wire format. However, this was not easy to integrate, so ContainerSSH 0.4 adds a field called publicKey containing the public key in the OpenSSH authorized keys format.

The publicKeyBase64 field is now removed because it was never useful. Authentication server implementations should switch to using the publicKey field.