style: use four-space indent size for go source files

[ijsong]

What this PR does

I talked with @hungryjang about the indent size of our project to set it to four spaces rather than
a tab.

[codecov-commenter]

Codecov Report

Base: 63.11% // Head: 63.09% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (54b6b83) compared to base (34fed84).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   63.11%   63.09%   -0.03%     
==========================================
  Files         130      130              
  Lines       17484    17484              
==========================================
- Hits        11035    11031       -4     
- Misses       5891     5892       +1     
- Partials      558      561       +3     

Impacted Files	Coverage Δ
pkg/varlog/subscribe.go	0.00% <0.00%> (ø)
internal/storagenode/replication_server.go	77.00% <0.00%> (-3.00%)	⬇️
internal/metarepos/storage.go	78.69% <0.00%> (-0.99%)	⬇️
internal/storagenode/logstream/executor.go	75.75% <0.00%> (-0.57%)	⬇️
internal/storagenode/storagenode.go	68.64% <0.00%> (+0.34%)	⬆️
internal/metarepos/report_collector.go	77.85% <0.00%> (+0.42%)	⬆️
...metarepos/dummy_storagenode_client_factory_impl.go	63.90% <0.00%> (+1.32%)	⬆️
internal/storagenode/logstream/committer.go	81.03% <0.00%> (+2.41%)	⬆️

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