Add a -providerPath option to jarsigner

When loading a keystore implementation not provided by JDK, keytool supports `-providerclass` and `-providerpath` to locate the implementation from a class file in the specified path. The jarsigner tool also supports `-providerClass` but no `-providerPath`. It should be added.

BTW, it's a tradition for keytool to define all lowercase option names and jarsigner mixed-case names, although both can load options in a case-insensitive way.