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

[bdealey]

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