search: add structural search service

Created by: rvantonder

Summary

• Adds structural searcher implementation. Comby is called when the searcher endpoint gets a request where patterntype:structural is set
• Adds tests for basic functionality

There are a bunch of things that need polishing that is not handled in this PR (like highlighting and handling other fields). These are omitted intentionally and will be handled over time. e2e integration tests to follow.

Created by: codecov[bot]

Codecov Report

Merging #6449 into master will decrease coverage by 0.14%. The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master    #6449      +/-   ##
==========================================
- Coverage   39.53%   39.38%   -0.15%     
==========================================
  Files        1192     1192              
  Lines       61568    61368     -200     
  Branches     5850     5850              
==========================================
- Hits        24341    24170     -171     
+ Misses      35021    34994      -27     
+ Partials     2206     2204       -2

Impacted Files	Coverage Δ
cmd/searcher/search/search_regex.go	89.68% <ø> (-3.61%)
cmd/searcher/search/search.go	67.53% <100%> (-6.89%)
cmd/searcher/search/search_structural.go	88.57% <88.23%> (+88.57%)
cmd/frontend/graphqlbackend/repository.go	21.84% <0%> (-0.98%)