fix: remove show tables column retrieval index based

[dpgaspar]

Improves the metadata retrieval of existing tables, the position based approach is bad and fails on elasticsearch 7.16.X.

Fixes: #80

[codecov-commenter]

Codecov Report

Merging #81 (b9f60ac) into master (cad716e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head b9f60ac differs from pull request most recent head 524cb28. Consider uploading reports for the commit 524cb28 to get more accurate results

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   94.39%   94.41%   +0.02%     
==========================================
  Files          15       15              
  Lines         963      967       +4     
==========================================
+ Hits          909      913       +4     
  Misses         54       54              

Flag	Coverage Δ
python	94.41% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
es/exceptions.py	100.00% <ø> (ø)
es/elastic/api.py	87.00% <100.00%> (+0.54%)	⬆️

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