feat(stdlib): add OptionShouldPing to control ResetSession ping behavior

[ilyam8]

Fixes #1738 (comment)

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In pgx/v5, the stdlib driver (database/sql integration) pings the server during Conn.ResetSession whenever a connection is reused after being idle for >1s. This matches pgxpool’s default ShouldPing logic, but for some proxies (e.g. PgBouncer admin console) the PostgreSQL -- ping command is not supported and results in noisy log entries such as:

invalid command '-- ping', use SHOW HELP;

Currently, stdlib users cannot disable this ping without setting MaxIdleConns(0) (which forces every query to open a new connection, causing connect/close churn).

This PR introduces an optional OptionShouldPing hook for stdlib, mirroring the existing pgxpool.ShouldPing design:

• If OptionShouldPing is not provided, default behavior remains unchanged: ping if idle > 1s.
• If provided, the hook decides whether a ping should be issued before reuse.