RA: Add GetAuthorization method to filter disabled challenges (#7652)

Add a new "GetAuthorization" method to the RA. This method is very
similar to the SA's existing "GetAuthorization2" method, except that it
also uses the RA's built-in Policy Authority to filter out any
challenges which are currently disabled.

In a follow-up change, the WFE will be updated to use this method when
retrieving authorizations and challenges for display, so that we can
ensure disabled challenges are not presented to ACME clients.

Part of #5913