Add support to read/write ACE restrictions from REST

Support for read/write of restrictions on the ACE has not yet been implemented in the sling jackrabbit.accessmanager REST operations. 
 
However it looks like adding support for read/write of ACE restrictions would not be too difficult and it looks like it could be useful to the community.
 
It looks like changes would be needed in a few bundles, specifically these:
 
1. org.apache.sling.jcr.base ( see SLING-8139  )
    a) Changes to the AccessControlUtil addEntry/replaceAccessControlEntry methods to support passing in and processing of restrictions
2. org.apache.sling.jcr.jackrabbit.accessmanager
    a) ModifyAce - changes to process posted "restriction" parameters to be set
    b) GetAcl/GetEffectiveAcl servlets - changes to return the declared restriction values in the returned JSON
    c) PrivilegesInfo - add methods to read the declared restrictions to be used from scripts who are interested in such things
3. Plus some automated tests and updated documentation...