feat(postgres): keepalives for tcp

[sargarass]

Does your PR solve an issue?

Situations where a server disconnects abruptly without sending a FIN packet

Is this a breaking change?

Yes

  Added                                                                                                                                                                                    
  - feat(postgres): TCP keepalive support — PostgreSQL connections now support TCP keepalive probes, matching libpq behavior. Keepalive is enabled by default (keepalives=1), consistent with libpq.

  - New PgConnectOptions methods:
    - keepalives(bool) / get_keepalives() — enable or disable TCP keepalives
    - keepalives_idle(Duration) / get_keepalives_idle() — idle time before probes begin
    - keepalives_interval(Duration) / get_keepalives_interval() — interval between probes
    - keepalives_retries(u32) / get_keepalives_retries() — max failed probes before dropping (Unix only)

  Changed
  - sqlx-core::net::connect_tcp now accepts an optional &KeepaliveConfig parameter to configure keepalive on the underlying TCP socket.
  - New dependency: socket2 0.5 (optional, enabled with _rt-tokio or _rt-async-std features) for cross-platform TCP keepalive configuration.

  Notes
  - MySQL connections pass None for keepalive (no keepalive configuration support yet).
  - Unix domain socket connections are unaffected — keepalive settings are ignored for UDS.
  - Zero values for timer parameters (keepalives_idle=0, keepalives_interval=0) are treated as "use OS default" and are not stored.

[sargarass]

Decided not to revive the existing PR and to just create a new one. For now my company have to use fork since we faced the same issue as acheung-harmonicinc described here