This client side plugin is used by a number of server plugins: LDAP (authentication_ldap_simple) and PAM (authentication_pam) to name a few.
The client name is mysql_clear_password
Client side requires nothing from the server. But the server generates and sends a 20-byte mysql_native_password compatible scramble.
Client side sends the password in clear text to the server

Note: Sending the scramble is not necessary for the clear text method, but, since the server always initiates the exchange by sending Protocol::Handshake and that one has a placeholder for authentication plugin dependent data the server does fill that space with a scramble should it come to pass that it will back down to mysql_native_password. This is also why it's OK no to specifically read this in clear_password_auth_client since it's already read as a part of the initial exchange.

See also: clear_password_auth_client, server_mpvio_write_packet, send_server_handshake_packet