Add TCP keepalive support for long queries

[sonnemusk]

Summary

Implements client-side TCP keepalive support for long-running queries (e.g. behind NATs, load balancers, or Redshift), addressing #360.

This supersedes the stale approach in #999 (2020). That PR wired net.Dialer.KeepAlive only (idle and interval forced to the same value, no count). This implementation:

• Uses libpq connection parameter names: keepalives, keepalives_idle, keepalives_interval, keepalives_count
• Applies settings after dial on *net.TCPConn via SetKeepAlive / SetKeepAliveConfig (Go 1.23+), so idle, interval, and count can be set independently
• Works with any dialer that returns a TCP connection (or a NetConn() wrapper such as *tls.Conn)
• Leaves dialer defaults alone when parameters are omitted; ignores Unix sockets / non-TCP
• Does not send keepalive keys as startup runtime parameters

Example:

postgres://…?keepalives=1&keepalives_idle=30&keepalives_interval=10&keepalives_count=5

Fields are also available on pq.Config.

Test plan

• Unit tests for DSN parsing (seconds → duration, count as int, invalid values, not leaked into Runtime)
• Unit tests for setTCPKeepalives against a real local TCP listener (enable/disable/tuning/non-TCP/no-op/NetConn unwrap)
• CI against lib/pq

Fixes #360