FFI: Allow restoring an existing session with SSS enabled.

[pixlwave]

Small follow-up to #3676 that makes 2 small changes:

• Ignore a Session's sliding sync proxy when restoring a client in the FFI so we don't have to do it in both EXI and EXA.
• Ignore a custom sliding sync proxy when building a client with support for SSS (not tested that one specifically but seemed worth adding whilst writing the other one).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.01%. Comparing base (dd20c37) to head (e644cb4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3723   +/-   ##
=======================================
  Coverage   84.01%   84.01%           
=======================================
  Files         260      260           
  Lines       26700    26700           
=======================================
  Hits        22432    22432           
  Misses       4268     4268           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Hywan]

Looks sane to me. Thanks!