feat: add hostname-suffix option to config-ssh (#17270)

Adds `hostname-suffix` as a Config SSH option that we get from Coderd, and also accept via a CLI flag.

It doesn't actually do anything with this value --- that's for PRs up the stack, since we need the `coder ssh` command to be updated to understand the suffix first.

cli/testdata/coder_config-ssh_--help.golden

@@ -33,6 +33,9 @@ OPTIONS:
           unix-like shell. This flag forces the use of unix file paths (the
           forward slash '/').
 
+      --hostname-suffix string, $CODER_CONFIGSSH_HOSTNAME_SUFFIX
+          Override the default hostname suffix.
+
       --ssh-config-file string, $CODER_SSH_CONFIG_FILE (default: ~/.ssh/config)
           Specifies the path to an SSH config.