[feat]: use pgconn.exec to execute multiple queries to eliminate roundtrip latency

[balajz]

Currently, when a user provides multiple queries and the configuration onError == resume is enabled, the entire statement block can be executed using conn.PgConn.Exec. The results can then be processed through multipleResultReader, instead of splitting the SQL queries and executing each statement individually. significantly reducing noticeable delay.

ref: jackc/pgx#1512