kvDurableTimeout needs to be implemented

The kvDurabilityTimeout mentioned in DOC-6075 and needed to be added in the Client Settings page, does not seem to be implemented in Go.

See the Scala implementation here: https://github.com/couchbase/couchbase-jvm-clients/commit/d2771f04437fe6681bf9bc3f42aea6c6ba577dc9

for SCBC-137: Add kvDurableTimeout

& Java:

https://github.com/couchbase/couchbase-jvm-clients/blob/master/core-io/src/main/java/com/couchbase/client/core/env/TimeoutConfig.java#L40

Docs in https://docs.couchbase.com/java-sdk/3.0/ref/client-settings.html#timeout-options-reference