fix: add writer custom endpoint type

[karenc-bq]

Summary

The AWS Java SDK may return WRITER:
https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/rds/model/CreateDbClusterEndpointResponse.html#customEndpointType()

Description

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[aaron-congo]

The SDK page you linked does seem to indicate that the WRITER type is possible, but this page seems to contradict that:
"Each custom endpoint has an associated type that determines which DB instances are eligible to be associated with that endpoint. Currently, the type can be READER or ANY."

Do we want to add this anyways?

[karenc-bq]

The SDK page you linked does seem to indicate that the WRITER type is possible, but this page seems to contradict that: "Each custom endpoint has an associated type that determines which DB instances are eligible to be associated with that endpoint. Currently, the type can be READER or ANY."

Do we want to add this anyways?

I am leaning towards us matching with whatever the API returns so we are not caught off guard when WRITER type endpoints are supported.