Simplify configs organization

[yurishkuro]

Description of the changes

• Move memstore config from pkg/memory to plugin/storage/memory, as the separation was unnecessary
• Simplify imports in the extension

How was this change tested?

• CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.40%. Comparing base (722755e) to head (86bd815).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5690      +/-   ##
==========================================
+ Coverage   96.38%   96.40%   +0.01%     
==========================================
  Files         334      334              
  Lines       16141    16141              
==========================================
+ Hits        15558    15560       +2     
+ Misses        405      404       -1     
+ Partials      178      177       -1     

Flag	Coverage Δ
badger_v1	8.04% <0.00%> (ø)
badger_v2	1.92% <0.00%> (ø)
cassandra-3.x-v1	16.60% <0.00%> (ø)
cassandra-3.x-v2	1.84% <0.00%> (ø)
cassandra-4.x-v1	16.60% <0.00%> (ø)
cassandra-4.x-v2	1.84% <0.00%> (ø)
elasticsearch-7.x-v1	18.88% <0.00%> (+0.01%)	⬆️
elasticsearch-8.x-v1	19.07% <0.00%> (ø)
elasticsearch-8.x-v2	19.07% <0.00%> (-0.02%)	⬇️
grpc_v1	9.46% <75.00%> (-0.02%)	⬇️
grpc_v2	7.49% <50.00%> (ø)
kafka	9.76% <100.00%> (ø)
opensearch-1.x-v1	18.93% <0.00%> (+0.01%)	⬆️
opensearch-2.x-v1	18.93% <0.00%> (ø)
opensearch-2.x-v2	18.93% <0.00%> (+0.01%)	⬆️
unittests	94.26% <100.00%> (+0.01%)	⬆️

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