nodemap: add a optional `nodemap_add_full` method on indexes

Authored by marmoute.


nodemap: add a optional nodemap_add_full method on indexes

This method can be used to obtains persistent data for a full nodemap. The end
goal is for some index implementation to managed the nodemap serialization them
selves (eg: the rust implementation)

Differential Revision: https://phab.mercurial-scm.org/D7841