feat: add CODER_CLUSTER_HOST CLI argument (#26680)

Closes GRU-69

Adds CODER_CLUSTER_HOST enviroment variable and CLI arg.

I ended up not making it hidden since we'll just have to unhide it later and even when hidden it still shows up in some autogenerated stuff. Might as well just go for it.

I also added it to the helm chart.

cli/testdata/coder_server_--help.golden

@@ -964,6 +964,9 @@ These options are only available in the Enterprise Edition.
       --browser-only bool, $CODER_BROWSER_ONLY
           Whether Coder only allows connections to workspaces via the browser.
 
+      --cluster-host string, $CODER_CLUSTER_HOST
+          Hostname or (more commonly) IP to reach this replica for clustering.
+
       --derp-server-relay-url url, $CODER_DERP_SERVER_RELAY_URL
           An HTTP URL that is accessible by other replicas to relay DERP
           traffic. Required for high availability.

cli/testdata/server-config.yaml.golden

@@ -190,6 +190,13 @@ networking:
   # Whether Coder only allows connections to workspaces via the browser.
   # (default: <unset>, type: bool)
   browserOnly: false
+  # Configure network clustering. Coder Servers in the primary region form a cluster
+  # by
+  # communicating directly.
+  cluster:
+    # Hostname or (more commonly) IP to reach this replica for clustering.
+    # (default: <unset>, type: string)
+    clusterHost: ""
 # Interval to poll for scheduled workspace builds.
 # (default: 1m0s, type: duration)
 autobuildPollInterval: 1m0s