meta: respond to ssh_info queries early

If the user opens the help screen early (i.e., when Subiquity starts
with a crash report), the installer will get stuck "Getting[ing] SSH
info". This is because the meta/ssh_info GET handler does not respond
until all the controllers have started.

Ensure it can respond early.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/common/apidef.py

@@ -120,6 +120,7 @@ def POST() -> None:
                 """Restart the server process."""
 
         class ssh_info:
+            @allowed_before_start
             def GET() -> Optional[LiveSessionSSHInfo]:
                 ...