add XDCR internal settings

curl -u Administrator:password -X GET http://127.0.0.1:9998/xdcr/internalSettings | jq '.'

curl -u Administrator:password -X POST http://127.0.0.1:9998/xdcr/internalSettings -d HealthCheckIntervalSec=5 | jq '.'

As per MB-47156,

the internal settings for xdcr are not documented. These should probably go into https://docs.couchbase.com/server/current/rest-api/rest-xdcr-adv-settings.html  with a pointer from rest-api/rest-get-internal-setting.html.