khepri_cluster: Add is_store_running/1 helper

[dumbbell]

The function verifies that:

1. the local Ra server is running
2. the store ID is known by the local instance of the application

[codecov]

Codecov Report

Base: 90.82% // Head: 90.79% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (4987dfd) compared to base (38f07ec).
Patch coverage: 90.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   90.82%   90.79%   -0.04%     
==========================================
  Files          19       19              
  Lines        3378     3388      +10     
==========================================
+ Hits         3068     3076       +8     
- Misses        310      312       +2     

Flag	Coverage Δ
erlang-24	89.49% <90.00%> (+<0.01%)	⬆️
erlang-25	89.63% <90.00%> (-0.09%)	⬇️
os-ubuntu-latest	90.67% <90.00%> (-0.10%)	⬇️
os-windows-latest	89.63% <90.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/khepri_cluster.erl	80.69% <90.00%> (+0.30%)	⬆️
src/khepri_machine.erl	94.67% <0.00%> (-0.14%)	⬇️

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