Allow clientauth extension to be used on a cluster with a Reader instance

## Describe the problem

* I have added a READER instance to my Aurora PostgreSQL v15 cluster, but users received an exception when the clientauth tried to insert/update on the client_lockout.failed_attempts table via the Reader instance.

## Describe the proposal

* Update the Insert/update command in the clientauth extension to catch the ReadOnly exception and log a special message to postgres/cloudwatch logs.
* I have submitted a PR from my fork with the proposed exception handling

## Describe alternatives

* Cannot currently use this extension on a cluster with Reader instances