Introduce new generic concept search endpoints

[cmark]

This PR introduces two new endpoints:

• GET /concepts
• POST /concepts/search

These endpoints allow generic concept search across all registered CodeSystems (similar to FHIR $lookup, but has many more options, like sorting, paging, term filtering, etc.).

[codecov]

Codecov Report

Attention: 29 lines in your changes are missing coverage. Please review.

Comparison is base (dc3211d) 64.08% compared to head (12c5e0c) 64.03%.
Report is 221 commits behind head on 8.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                8.x    #1172      +/-   ##
============================================
- Coverage     64.08%   64.03%   -0.06%     
- Complexity    12775    12787      +12     
============================================
  Files          1763     1765       +2     
  Lines         59585    59668      +83     
  Branches       5497     5512      +15     
============================================
+ Hits          38184    38206      +22     
- Misses        19032    19086      +54     
- Partials       2369     2376       +7     

Files	Coverage Δ
...nal/snowowl/core/request/ConceptSearchRequest.java	78.72% <40.00%> (-5.00%)	⬇️
.../snowowl/core/rest/concepts/ConceptRestSearch.java	0.00% <0.00%> (ø)
...l/core/rest/concepts/ConceptSearchRestService.java	13.33% <13.33%> (ø)

... and 19 files with indirect coverage changes

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[apeteri]

LGTM!

[nagyo]

LGTM! 🍏