Add create_shards function

[knizhnik]

No description provided.

[funbringer]

Тесты не прошли, но это не важно. Я, наверное, возьму отдельно фикс по поводу inheritance, а остальное у меня есть (или скоро будет) в pathman_sharding.

[codecov-io]

Codecov Report

Merging #85 into master will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   90.43%   90.63%   +0.19%     
==========================================
  Files          23       23              
  Lines        4527     4527              
  Branches      729      729              
==========================================
+ Hits         4094     4103       +9     
+ Misses        433      424       -9

Impacted Files	Coverage Δ
src/planner_tree_modification.c	96.17% <0%> (+1.09%)	⬆️
src/init.c	93.69% <0%> (+1.26%)	⬆️
src/relation_info.c	91.17% <0%> (+1.47%)	⬆️

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