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.

docs/reference/cli/server.md

@@ -1122,6 +1122,16 @@ The algorithm to use for generating ssh keys. Accepted values are "ed25519", "ec
 
 Whether Coder only allows connections to workspaces via the browser.
 
+### --cluster-host
+
+|             |                                             |
+|-------------|---------------------------------------------|
+| Type        | <code>string</code>                         |
+| Environment | <code>$CODER_CLUSTER_HOST</code>            |
+| YAML        | <code>networking.cluster.clusterHost</code> |
+
+Hostname or (more commonly) IP to reach this replica for clustering.
+
 ### --scim-auth-header
 
 |             |                                      |