Add testing for 6.5 with DP enabled

As enabling the developer features in 6.5 significantly impacts the behaviour of the server and SDKs in terms of performing operations, the SDKs should explicitly test against the 6.5 server with preview mode enabled in integration tests.

Note that there are significant known issues with the use of services such as N1QL when developer preview is enabled and thus it is expected that each SDK has some method to signal whether the developer preview is enabled in tests (this flag would disable N1QL for 6.5 as an example).