Introduce stubs for Raft cluster bus

[murphyjacob4]

Following the cluster bus separation, introduce the stubs for a Raft-based cluster bus.

[codecov]

Codecov Report

❌ Patch coverage is 1.43885% with 137 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.47%. Comparing base (6b93134) to head (4be1c71).

Files with missing lines	Patch %	Lines
src/cluster_raft.c	0.00%	136 Missing ⚠️
src/cluster.c	50.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           cluster-v2    #3530      +/-   ##
==============================================
- Coverage       76.59%   76.47%   -0.13%     
==============================================
  Files             160      161       +1     
  Lines           79288    79427     +139     
==============================================
+ Hits            60733    60743      +10     
- Misses          18555    18684     +129     

Files with missing lines	Coverage Δ
src/config.c	78.33% <ø> (ø)
src/server.c	89.59% <100.00%> (+0.03%)	⬆️
src/server.h	100.00% <ø> (ø)
src/cluster.c	90.60% <50.00%> (-0.18%)	⬇️
src/cluster_raft.c	0.00% <0.00%> (ø)

... and 20 files with indirect coverage changes

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