define the interface for merging indexes

[inabao]

#318

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/engine.cpp	84.37%	5 Missing ⚠️
src/index/hnsw.cpp	93.93%	2 Missing ⚠️
src/index/hnsw_test.cpp	98.07%	1 Missing ⚠️

@@            Coverage Diff            @@
##           main     #312       +/-   ##
=========================================
+ Coverage      0   88.18%   +88.18%     
=========================================
  Files         0      146      +146     
  Lines         0     9462     +9462     
=========================================
+ Hits          0     8344     +8344     
- Misses        0     1118     +1118     

Flag	Coverage Δ
cpp	88.18% <94.59%> (?)

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Files with missing lines	Coverage Δ
src/algorithm/hnswlib/hnswalg.cpp	87.34% <100.00%> (ø)
src/algorithm/hnswlib/hnswalg.h	96.87% <ø> (ø)
src/index/hnsw.h	100.00% <ø> (ø)
src/merge_index.cpp	100.00% <100.00%> (ø)
src/index/hnsw_test.cpp	98.54% <98.07%> (ø)
src/index/hnsw.cpp	90.74% <93.93%> (ø)
src/engine.cpp	89.79% <84.37%> (ø)

... and 139 files with indirect coverage changes