Topology Service CellService client

What does this MR do and why?

Topology Service CellService client

Addressing: #496760 (closed)

How to set up and validate locally

1. Check out this branch 496760-topology-service-module-cells-service-class.
2. Make sure you have Topology Service enabled in GDK gdk set gitlab_topology_service.enabled true
3. Enable Topology Service in the Rails application. Make sure you have this in your gdk.yml

gitlab:
  topology_service:
    enabled: true
    address: 127.0.0.1:9095

4. Run gdk reconfigure
5. in gdk/gitlab-topology-service, make sure you have cell-0 in your config.toml. GDK doesn't add this yet. You can have these for the cells

[[cells]]
id = 1
name = "cell-0"
address = "gdk.test:12001"
session_prefix = "cell_0_"
sequence_range = [0, 4398046511104]


[[cells]]
id = 100
name = "cell-1"
address = "gdk.test:12001"
session_prefix = "cell_1_"
sequence_range = [0, 4398046511104]

6. Restart Topology Service gdk restart gitlab-topology-service
7. Run this from your Rails Console

Gitlab::TopologyServiceClient::CellService.new.get_cell_info

You should get something like

=> <Gitlab::Cells::TopologyService::CellInfo: id: 1, name: "cell-0", address: "gdk.test:12001", session_prefix: "cell_0_", sequence_range: <Gitlab::Cells::TopologyService::SequenceRange: minval: 0, maxval: 4398046511104>>

Related to #496760 (closed)