Refactor gRPC storage plugin for better composability

[yurishkuro]

Allow reusing Remote Storage GRPCHandler without going through plugin APIs

[codecov]

Codecov Report

Merging #3833 (e3469cc) into main (faf4bc9) will decrease coverage by 0.00%.
The diff coverage is 72.10%.

@@            Coverage Diff             @@
##             main    #3833      +/-   ##
==========================================
- Coverage   97.58%   97.58%   -0.01%     
==========================================
  Files         291      291              
  Lines       16939    16962      +23     
==========================================
+ Hits        16530    16552      +22     
  Misses        323      323              
- Partials       86       87       +1     

Impacted Files	Coverage Δ
plugin/storage/grpc/memory/plugin.go	100.00% <ø> (ø)
plugin/storage/grpc/shared/plugin.go	0.00% <0.00%> (ø)
plugin/storage/grpc/shared/grpc_handler.go	74.05% <74.05%> (ø)

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