pgcrypto: make expensive functions respect context cancellation

**Is your feature request related to a problem? Please describe.**
When using the crypto gen_salt function setting the iter_count to the higher end of acceptable values can take a very long time to complete. While these values aren't normally in use, if an operator does decide to use one, then this can lock the cluster into a pretty resource intensive operation until it completes or the cluster is restarted.

**Describe the solution you'd like**
Ideally we would be able to interrupt these functions. It seems like the two possible solutions are to fork bcrypt or to wait for something [like this pull request ](https://github.com/golang/go/issues/56416) to be released in a newer version of go.

Jira issue: CRDB-31750

Epic CRDB-60810