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 <[email protected]>
(cherry picked from commit 41abc35)

subiquity/common/apidef.py

@@ -126,6 +126,7 @@ def POST() -> None:
                 """Restart the server process."""
 
         class ssh_info:
+            @allowed_before_start
             def GET() -> Optional[LiveSessionSSHInfo]:
                 ...