Support RFC5656

Hello. I’d like to ask for adding support for RFC 5656.

KEX algorithm:
ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521

Host Key:
ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521

Thanks.