feat: NamedQueryContext on *sqlx.Tx

[sonnemusk]

Summary

*sqlx.Tx already has NamedQuery and NamedExecContext, but was missing NamedQueryContext (issue #447).

tx, err := db.BeginTxx(ctx, nil)
rows, err := tx.NamedQueryContext(ctx,
    `SELECT * FROM person WHERE first_name=:first_name`, person)

Thin wrapper around the existing package-level NamedQueryContext, matching *DB.

Fixes #447