Add a new option 'pqopt' which specifies libpq connection parameters as a conninfo...

Add a new option 'pqopt' which specifies libpq connection parameters as a conninfo style string. The string must be enclosed with braces {} e.g. pqopt={sslcert=... sslkey=... sslrootcert=...} in the case of connection strings. Though host, port, dbname, user, password, sslmode, keepalives_idle or keepalive_interval parameters can be set using this option, they are ordinarily set by other options. When some settings for those parameters conflict with other ordinary options, connections are rejected.