* Forbid ssh key signing with specified extensions when role allowed_extensions is not set
- This is a behaviour change on how we process the allowed_extensions role
parameter when it does not contain a value. The previous handling allowed
a client to override and specify any extension they requested.
- We now require a role to explicitly set this behaviour by setting the parameter
to a '*' value which matches the behaviour of other keys such as allowed_users
within the role.
- No migration of existing roles is provided either, so operators if they truly
want this behaviour will need to update existing roles appropriately.
