Add SeedNode for custom ports per node

Java has the concept of a "seed node" that allows custom ports to be added per node.

• https://github.com/couchbase/couchbase-jvm-clients/blob/master/core-io/src/main/java/com/couchbase/client/core/env/SeedNode.java#L71
• https://github.com/couchbase/couchbase-jvm-clients/commit/3e5986ee506f35f2fb6e85f9d286f17138b0b888#diff-dd5ac6ec174d442149876c54efff9058
• Using the connection string, this is exposed like this: [couchbase://foo:1234]=manager,foo:5678=kv

While not asked for yet, this is something that will likely come up in containerized environments.