opensearch: increase test coverage

[Jakob3xD]

Description

Depends on #512 to be merged first.

• removed unused structs and functions from opensearch
• adjust opensearch test to increase coverage
• fix opensearchtransport to check for empty response body

Issues Resolved

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.24%. Comparing base (06a6dc8) to head (8a15689).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #517      +/-   ##
==========================================
+ Coverage   57.29%   63.24%   +5.94%     
==========================================
  Files         315      315              
  Lines        9823     7715    -2108     
==========================================
- Hits         5628     4879     -749     
+ Misses       2902     1550    -1352     
+ Partials     1293     1286       -7     

Flag	Coverage Δ
integration	55.78% <0.00%> (+4.94%)	⬆️
unit	81.45% <100.00%> (+68.60%)	⬆️

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Files	Coverage Δ
opensearch.go	92.72% <100.00%> (+17.10%)	⬆️
opensearchtransport/discovery.go	64.70% <100.00%> (+1.62%)	⬆️

... and 248 files with indirect coverage changes