(PE-4617) JVM Cert authority can now retrieve decoded extension values

For the code where you're ultimately going to use this stuff, do you end up combining the critical and not-critical extensions together?  If so, would it make sense to just combine them here so there's only one `getExtensions` function to plumb up the stack -- Java -> Clojure.  This could eliminate the need to explain what "critical" vs. "non-critical" means.