Pass null-terminated node ID for VM_RegisterClusterMessageReceiver and add test coverage

[Nikhil-Manglore]

• Pass sender_id as NULL terminated string as part of ValkeyModuleClusterMessageReceiver for ease of usage by the module(s).

• Implement test coverage described in Add test coverage for module callback registration for cluster message via VM_RegisterClusterMessageReceiver #1656 and ensures that nodes in the cluster module properly acknowledge a "DING" message by sending a "DONG" response.

Closes #1656.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 71.14%. Comparing base (da3f1c6) to head (18117ba).
Report is 37 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/cluster_legacy.c	0.00%	3 Missing ⚠️
src/module.c	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1708      +/-   ##
============================================
+ Coverage     71.06%   71.14%   +0.08%     
============================================
  Files           121      123       +2     
  Lines         65254    65534     +280     
============================================
+ Hits          46371    46627     +256     
- Misses        18883    18907      +24     

Files with missing lines	Coverage Δ
src/module.c	9.61% <0.00%> (+0.01%)	⬆️
src/cluster_legacy.c	85.87% <0.00%> (-0.39%)	⬇️

... and 32 files with indirect coverage changes

[Nikhil-Manglore]

Looking into the CI Failure

[hpatro]

Mostly LGTM.

[zuiderkwast]

Yeah, just some nits, then let's merge it.