Consider handling DNS errors as transient

We've gotten a [report](https://github.com/npgsql/efcore.pg/issues/3248) that in certain environments, a DNS error can be transient, and therefore needing a retry (although in most normal situation it isn't). We still need to understand this better and see whether other people run into issues around this (and on which databases).

In the meantime, it should be trivial for users to extend the provider's execution strategy (e.g. NpgsqlRetryingExecutionStrategy) to make it treat DNS exceptions as transient.