APIs to generate various String references from a UserReference

Right now we have only a single `UserReferenceSerializer<String>` implementation per user storage type. It seems we might need more than one.

Context:

• https://github.com/xwiki/xwiki-platform/commit/6f25df12c19a76f2e364471842de7bda363b4320#commitcomment-67145551
• In short in XWiki we store the user reference using a "compactwiki" serialized to not save the wiki part if it's the same as the current wiki. This allows for an easy migration of DB schemas.

Discussion:

• https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$16454509741128lbgWw:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=mephis.to

Vincent:

For the future, we could imagine a "local" or "global" string serialization I guess (for LDAP, "global" would mean adding the ou or dcpart
ex: "cn=Joe Smith,ou=East,dc=MyDomain,dc=com"
global = "cn=Joe Smith,ou=East,dc=MyDomain,dc=com"
local = "cn=Joe Smith"
not sure though
just an idea
the harder part is that "compactwiki" is not the same as current
(I'm not sure what the need for "local" LDAP string would be though )
I guess for the same reason, for example you want to save the user ref somewhere, independently of its organizational unit (or domain)
We could also have a generic "store" hint for "UserRefefenceSerializer" to generate a string reference that can be used to store user references for easy moves/renames/etc