ldb: Allow to register extended match rules

This allows to extend LDB by registering extended match rules from outside
the library itself. This is necessary when the implementation requires
knowledge about syntaxes implemented in samba extensions, like the
LDAP_MATCHING_RULE_TRANSITIVE_EVAL match.

Signed-off-by: Samuel Cabrero <samuelcabrero@kernevil.me>
Singed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>