rds proxy support for postgres connections

[tmm1]

cc prisma/prisma#5866 prisma/prisma#23547

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[tmm1]

After some testing, the best configuration is to simply use the client against RDS proxy as-is.

It is definitely not recommended to add ?pgbouncer=true, as that will both increase QPM (due to disabled statement caches) and will cause connection-pinning (due to use of DEALLOCATE ALL).