sql/auth: support for sslnegotation=direct

Postgres 17 adds sslnegotation=direct (defaults to sslnegotation=postgres which asks the server if TLS is supported).
> This option controls how SSL encryption is negotiated with the server, if SSL is used. In the default `postgres` mode, the client first asks the server if SSL is supported. In `direct` mode, the client starts the standard SSL handshake directly after establishing the TCP/IP connection. Traditional PostgreSQL protocol negotiation is the most flexible with different server configurations. If the server is known to support direct SSL connections then the latter requires one fewer round trip reducing connection latency and also allows the use of protocol agnostic SSL network tools. The direct SSL option was introduced in PostgreSQL version 17.

https://www.postgresql.org/docs/17/libpq-connect.html#LIBPQ-CONNECT-SSLNEGOTIATION

Jira issue: CRDB-42629

Epic CC-33751