Allow storing extensions in a shared location

It would make easier to support extension sharing in a cluster (so it would fix XWIKI-11441), and it's required for XWIKI-22847.

One general problem with extensions, and the reason it was put in the filesystem in the first place, is that the database init is way too late for various types of extensions. Filesystem was simply the least setup-demanding (and so the earliest) way to load extensions.

There are actually two difference pieces of information to store (which may or may not be stored in the same location):

• the local (already downloaded) extensions (extension descriptor and file)
• the index of installed extension