Passing authorization credentials to REST running on same application instance

I am developing an confluence application where there are 2 REST plugins A and B, both exposes their respective REST end points. Rest A consumes Rest B. I want to pass the current user credentials from A to B. How? REST B can be POST, GET, PUT or DELETE. How? I have considered following options unsuccessfully.

  1. Shared Access Layer - this apparently used to work with the method Request#addTrustedTokenAuthentication() but is deprecated in SAL 3.0.5, see SAL Documentation (outdated?), and SAL Version Matrix
  2. ApplicationLink - would allow me to link to another application, but apparently it’s not possible to link back to the same Confluence instance
  3. SAL TrustedRequestFactory - seems to be depricated. Also does it support POST?