Implement ECDSA code for handling signature requests in daemon/ssh-agent. This does not include actually doing the signing but parsing the requests and keys that come in on ssh-agent binary protocol, and validating them and passing them through to the PKCS#11 key store.

Reference for how to implement this would be the openssh sources.

Functions that are probably affected:
 * build_like_attributes()
 * op_add_identity()
 * op_sign_request()
 * gkd_ssh_agent_initialize_with_module()
 * gkd_ssh_agent_proto_keytype_to_algo()
 * gkd_ssh_agent_proto_algo_to_keytype()
 * gkd_ssh_agent_proto_read_public()
 * gkd_ssh_agent_proto_write_public()

Possible new functions, similar to functions for other algorithms nearby:
 * gkd_ssh_agent_proto_read_pair_ecdsa()
 * gkd_ssh_agent_proto_read_public_ecdsa()
 * gkd_ssh_agent_proto_write_public_ecdsa()
 * gkd_ssh_agent_proto_write_signature_ecdsa()