Allow adding InternalPrivateKeyManagers together with their corresponding...

Allow adding InternalPrivateKeyManagers together with their corresponding public key managers to the registry.

We require public key manager at the same time because we need to get the key type. Doing this also has an advantage to the programmer: the compiler ensures that the public key types of the two given objects match, so I actually prefer this interface.

PiperOrigin-RevId: 255166289