feat: support --hostname-suffix flag on coder ssh (#17279)

Adds `hostname-suffix` flag to `coder ssh` command for use in SSH Config ProxyCommands.

Also enforces that Coder server doesn't start the suffix with a dot.

part of: #16828

cli/testdata/coder_ssh_--help.golden

@@ -23,6 +23,11 @@ OPTIONS:
           locally and will not be started for you. If a GPG agent is already
           running in the workspace, it will be attempted to be killed.
 
+      --hostname-suffix string, $CODER_SSH_HOSTNAME_SUFFIX
+          Strip this suffix from the provided hostname to determine the
+          workspace name. This is useful when used as part of an OpenSSH proxy
+          command. The suffix must be specified without a leading . character.
+
       --identity-agent string, $CODER_SSH_IDENTITY_AGENT
           Specifies which identity agent to use (overrides $SSH_AUTH_SOCK),
           forward agent must also be enabled.