Add Cloud Native Gateway Support to XDCR

Cloud Native Gateway will be available in CAO 2.6.1, with SDK support in Java 3.5 and Go 2.7.

To allow customers to replicate in to a cluster that is only exposing CNG, support will need to be added to XDCR.

 
Currently, there are two use cases for XDCR with CNG:

• XDCR over CNG with private links
• XDCR over CNG in kubernetes

 
The basic requirement is that XDCR over CNG, from the user perspective, work as it currently does – i.e. just give the correct (CNG) connection string (maybe port), credentials, and security certificate to create the remote target reference.
 
The only user-facing change is that they need to enter the correct connection string if they are connecting to a CNG-fronted cluster.  The credentials and security certificate should be handled in the same way as existing, but the connection string becomes “couchbase2”.  There should be a default port (18098), which the user would need to provide if they have changed.
 
Note: "couchbase2" is documented in client settings configuration for connecting to a Couchbase Server cluster – for example – https://docs.couchbase.com/kotlin-sdk/current/howtos/connecting.html#cloud-native-gateway