Get column names for `db_postgres`?

[sdmcallister]

instantRows should provide access to the columns: `iterator instantRows(db: 
DbConn; columns: var DbColumns; query: SqlQuery; args: varargs[string, `$`]): 
InstantRow {...}`

Here is the example from the db_sqlite 
    
    
    var columns: DbColumns
    for row in db.instantRows(columns, sql"SELECT * FROM my_table"):
      discard
    echo columns[0]
    
    # Output:
    # (name: "id", tableName: "my_table", typ: (kind: dbNull,
    # notNull: false, name: "INTEGER", size: 0, maxReprLen: 0, precision: 0,
    # scale: 0, min: 0, max: 0, validValues: @[]), primaryKey: false,
    # foreignKey: false)
    
    
    Run