Implement command 0x41 which is used by OpenSSH for reading RKs. It has
the following subcommands:
* CMD_CRED_METADATA - get number of saved/remaining RKs
* CMD_RP_BEGIN/CMD_RP_NEXT - iterate over the saved RPs
* CMD_RK_BEGIN/CMD_RK_NEXT - iterate over the RKs for a given RP
Fixes issue #374 and issue #314