feat(scheduler-bindings): external scheduler handshake thread

[OliverNChalk]

Problem

• External schedulers must connect & configure their session via IPC (includes exchanging shared memory FDs).

Summary of Changes

• Spawn a dedicated thread to block on new scheduler connections.

Fixes #

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (8a94aa3) to head (7b96f2c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8817     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         864      865      +1     
  Lines      375142   375161     +19     
=========================================
- Hits       312355   312348      -7     
- Misses      62787    62813     +26     

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