feat(fhir.core): Add generic "constraint" filter

[apeteri]

It is handled the same way as SNOMED CT's "expression" filter defined on the FHIR code system resource:

{
  "code" : "expression",
  "description" : "The result of the filter is the result of executing the given SNOMED CT Expression Constraint",
  "operator" : ["="],
  "value" : "A SNOMED CT ECL expression (see https://snomed.org/ecl)"
}

...but is applicable to any code system tooling with ECL support.

[codecov-commenter]

Codecov Report

Base: 64.25% // Head: 64.25% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (beaa0d7) compared to base (6c37cd5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##                8.x    #1063      +/-   ##
============================================
- Coverage     64.25%   64.25%   -0.01%     
+ Complexity    12437    12436       -1     
============================================
  Files          1732     1732              
  Lines         58045    58056      +11     
  Branches       5361     5361              
============================================
+ Hits          37298    37304       +6     
- Misses        18446    18450       +4     
- Partials       2301     2302       +1     

Impacted Files	Coverage Δ
...al/snowowl/fhir/core/model/codesystem/Filters.java	98.55% <100.00%> (+0.16%)	⬆️
...ernational/snowowl/core/request/DeleteRequest.java	76.92% <0.00%> (-6.42%)	⬇️
...owl/core/identity/AuthorizationHeaderVerifier.java	55.26% <0.00%> (-4.74%)	⬇️
...index/src/com/b2international/index/es/EsNode.java	81.25% <0.00%> (-1.05%)	⬇️

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[cmark]

🥓